Hi,Please forgive my ignorance, but I'm still having difficulty understanding memory usage etc.I use Vista 64 on a 64 bit motherboard so both recognise my 4G RAM - no problem there.Do I still need to modify the FSX.exe and FS9.exe header so that the applications can use the 4G?ThanksPaul

Hum.. How long have you got.32bit OS By default any one process (FS9) can only use 2Gbyte of memory,and the entire system can only use 4Gbyte of memory. When I saymemory this is virtual memory - though can (in your case) be physicalmemory. So when you run FS9 without an OS tweak your process willonly get 2G before you get an OOM. The problem is that FS9(X) canuse lots of memory when you have complex airports/scenery/planeseven when they have not hit the empty texture folder problem. So what happens with the last 2G of memory : the system uses itfor a shadow of the graphics card memory, and all the other processesyou run can use it (realplayer and other crapware, anti-virus andvirus when you don't bother with the former) AIsmooth, ActiveSky,etc. OK : you can tweak the OS to get 3Gbyte allocation to any oneprocess. This is often used for servers with limited applicationson it, though I have seen posts from people that have used this tweak.If you are asking this question then I recommend you don't do the following. Change your boot.ini file and add a 3G switch to theOS boot line. This will modify the system to allow FS9 with aLARGEADRESS setting to take more space (3G). It also means thatmany other applications will fail (McAFee AntiVirus is one, butmany Anti-virus are very picky about what the system looks like). You now only have 1G left for the system - so what when all goespear shapped cos you have a dual large memory SLI Nvida cards thathave a total GPU memory greater than 1G. Think carefully beforeyou jump this route.64bit OS on 64 bit architecture chipsWell you can get 64bit XP and 64bit Vista. Normally programs needto be recompiled to run on 64 bit OS, then you can address stupidlarge amounts of memory. But since MS don't supply the source codeand compiler you make do with a cludge that allows 32bit compiledcode to run on 64bit machines - and you get (wait for it) another2Gbytes - so all applications can 4Gbytes of memory. The problemhere is that 64bit OS is still not really the games market so many device drivers and stuff will not work with 64bit OS. Ofcourse you will need to by 64bit XP/Vista, get new device driversand check all your hardwareOK, so can you use your 4G for fs9 ?1) You current setup : No2) Using 3G switch : No but you can use 3G - but you may render your computer unusable3) Buy 64 bit OS : yes.But your computer will be using some of this memory, and if youhave large GPU memory then I guess this will not get swapped out.