http://blogs.msdn.com/oldnewthing/archive/.../01/423817.aspx"it can afford to give the 32-bit Windows program the entire 4GB of address space to use."To sum up: - XPSP2 added PAE support, but still doesn't go beyond 4GB - Memory belonging to PCI devices (e.g. video memory) is mapped below the 2^32 border - 32-bit Windows 2003 Server (and Longhorn Server) can and will use memory above the 2^32 border (but most/all nVidia drivers after 79.11 will not work reliably -- bad drivers like these is the main reason MS chose not to take full advantage of PAE in XP) - 32-bit processes (with the correct flag set in the PE header) do indeed enjoy a 4GB virtual address space when run within a 64-bit OS

I see there is a great deal of confusion in this post, so let me see if I can clear some things up. First, the HardOCP post does go into great detail over the limitations of 32-bit Windows (both XP and Vista) and is quite correct. Unfortunately if you don't understand how operating systems manage memory, it may be a bit much to digest. I'll try to explain it as simply as I can, but first, a few things to get out of the way:1) 32-bit Windows, both Vista and XP, can only use up to 3.2 Gigabytes of physical RAM at once for all running programs. This is a hard limit due to the nature of a 32-bit OS, and due to the nature of how Windows maps memory.2) Moving to 32-bit Vista does not solve this limitation, as it is still a 32-bit OS. See 1) above.3) Moving to a 64-bit OS does solve the problem. There is a lot of FUD about 64-bit Windows, especially Vista. Some of this is warranted, while much of it isn't. Personally, I have a had a great deal of experience with both 64-bit XP and Vista, and in mine and many other's opinion, Vista 64 is the better OS. More on this later.Now, to clarify some of the confusion:The 3.2 limit is a maximum limit regarding how much physical memory the OS can see and use at once. In reality, this limit is determined by the amount of component memory installed in your system, such as the memory in your video card. 32-bit Windows has only one 4-gig address space, and it maps component memory before your main system RAM. Therefore, the actual limit is 4 gigs minus minus your PCI/PCI-E/BIOS/etc. memory. Even if you have less than ~800Megs, Windows will still reserve this amount which gives us the max limit of 3.2 gigs for 32-bit Windows.64-bit Windows does not share this problem because a 64-bit address space is orders of magnitude larger than a 32-bit address space. The limits placed on the amount of RAM you can have in 64-bit Windows XP and Vista are not based on address space limits, but are either arbitrary or due to other technical limitations. Now, as there are limits on how much memory the entire OS can use at once, there are also limits on how much memory a single program can use at once out of the upper OS limit. In a sense, if the 3.2 limit determines how big a pizza pie can be, then the program memory limit determines how large a slice can be in that 3.2 gig pie. For 32-bit programs, that limit is 2 gigs. However, as explained in the HardOCP post, there is a compiler switch, which can be applied to an individual program, which can increase the slice to 3 Gigs. 64-bit programs have, naturally, a much larger max slice size.Lastly, there is also an esoteric switch called PAE which can, in some instances, allow 32-bit Windows to see all 4 gigs of RAM, hence making the actual pie bigger for the 32-bit OS. However, it is important to note that this switch is known to be very buggy causing instability problems and performance decreases on desktop machines. It is used successfully on limited server environments, but if you have four gigs and want to use it all on your desktop, the only option worth considering is a 64-bit OS.Now, 64-bit XP has always been an odd duck, as its really Windows Server 2003 with XP dressing. Additionally, it came out well after XP-32 and has never received much fanfare or publicity. As as result, it's always been an awkward os, with little 3rd party driver and software support. It's userbase is also quite small, so if something doesn't work, it isn't as easy to find people to help fix the problem.Vista 64, on the other hand, is most certainly a first-tier brother of the 32-bit version. Sure, while Vista 64 shares some of the rough edges of Vista-32, these edges are constantly being smoothed by patches and improved driver releases. This is just the nature of a new OS. XP-64 on the other-hand, will always remain awkward, as there is little demand these days for the OS, and as such, little incentive to support it. Furthermore, in mine any people's experiences, Vista 64 runs better than Vista-32 on a well equipped system. This is likely due to the 64-bit OS having a more mature code base that doesn't need to support old, troublesome hardware. Bottom line, if you want four gigs of RAM to be used, and you're ready to take the plunge into Vista, then by all means upgrade to Vista 64. Chances are you'll be glad you did. If you would rather stay with XP, then either let the last gig sit there unused and stay with 32, or return it. Good luck,Mark.

I see there is a great deal of confusion in this post, so let me see if I can clear some things up. First, the HardOCP post does go into great detail over the limitations of 32-bit Windows (both XP and Vista) and is quite correct. Unfortunately if you don't understand how operating systems manage memory, it may be a bit much to digest. I'll try to explain it as simply as I can, but first, a few things to get out of the way:1) 32-bit Windows, both Vista and XP, can only use up to 3.2 Gigabytes of physical RAM at once for all running programs. This is a hard limit due to the nature of a 32-bit OS, and due to the nature of how Windows maps memory.2) Moving to 32-bit Vista does not solve this limitation, as it is still a 32-bit OS. See 1) above.3) Moving to a 64-bit OS does solve the problem. There is a lot of FUD about 64-bit Windows, especially Vista. Some of this is warranted, while much of it isn't. Personally, I have a had a great deal of experience with both 64-bit XP and Vista, and in mine and many other's opinion, Vista 64 is the better OS. More on this later.Now, to clarify some of the confusion:The 3.2 limit is a maximum limit regarding how much physical memory the OS can see and use at once. In reality, this limit is determined by the amount of component memory installed in your system, such as the memory in your video card. 32-bit Windows has only one 4-gig address space, and it maps component memory before your main system RAM. Therefore, the actual limit is 4 gigs minus minus your PCI/PCI-E/BIOS/etc. memory. Even if you have less than ~800Megs, Windows will still reserve this amount which gives us the max limit of 3.2 gigs for 32-bit Windows.64-bit Windows does not share this problem because a 64-bit address space is orders of magnitude larger than a 32-bit address space. The limits placed on the amount of RAM you can have in 64-bit Windows XP and Vista are not based on address space limits, but are either arbitrary or due to other technical limitations. Now, as there are limits on how much memory the entire OS can use at once, there are also limits on how much memory a single program can use at once out of the upper OS limit. In a sense, if the 3.2 limit determines how big a pizza pie can be, then the program memory limit determines how large a slice can be in that 3.2 gig pie. For 32-bit programs, that limit is 2 gigs. However, as explained in the HardOCP post, there is a compiler switch, which can be applied to an individual program, which can increase the slice to 3 Gigs. 64-bit programs have, naturally, a much larger max slice size.Lastly, there is also an esoteric switch called PAE which can, in some instances, allow 32-bit Windows to see all 4 gigs of RAM, hence making the actual pie bigger for the 32-bit OS. However, it is important to note that this switch is known to be very buggy causing instability problems and performance decreases on desktop machines. It is used successfully on limited server environments, but if you have four gigs and want to use it all on your desktop, the only option worth considering is a 64-bit OS.Now, 64-bit XP has always been an odd duck, as its really Windows Server 2003 with XP dressing. Additionally, it came out well after XP-32 and has never received much fanfare or publicity. As as result, it's always been an awkward os, with little 3rd party driver and software support. It's userbase is also quite small, so if something doesn't work, it isn't as easy to find people to help fix the problem.Vista 64, on the other hand, is most certainly a first-tier brother of the 32-bit version. Sure, while Vista 64 shares some of the rough edges of Vista-32, these edges are constantly being smoothed by patches and improved driver releases. This is just the nature of a new OS. XP-64 on the other-hand, will always remain awkward, as there is little demand these days for the OS, and as such, little incentive to support it. Furthermore, in mine and many people's experiences, Vista 64 runs better than Vista-32 on a well equipped system. This is likely due to the 64-bit OS having a more mature code base that doesn't need to support old, troublesome hardware. Bottom line, if you want four gigs of RAM to be used, and you're ready to take the plunge into Vista, then by all means upgrade to Vista 64. Chances are you'll be glad you did. If you would rather stay with XP, then either let the last gig sit there unused and stay with 32, or return it. Good luck,Mark.

Running FSX on Vista-64 bit with FSX-SP1 patch. Vista lists performance rating at 5.9. It see's all 4 gig of ram.Runs like FS9 with ALL sliders etc., maxed.ACES did a good job.Keep the ram. If you send it back you may not find new ram that will play nice with what you now have. They seem to like to change modual's. Even lot numbers can sometimes make a difference if they use another manufacturer.Go to Corsair message boards, see what I mean.