Lots of discussion on this in the FSX forum--is there any performance gain in FS9 with a dual-core with this fix? Apparently when you have a dual core AMD you have to download AMD patch first. Here's the excerpt from the microsoft download page:"SYMPTOMSComputers that are equipped with multiple processors that support processor power management features, such as Advanced Configuration and Power Interface (ACPI) processor performance states, require Microsoft Windows XP Service Pack 2 (SP2). Additional updates are available to optimize performance and behavior on computers that are running Windows XP SP2. Without these updates, computers that are equipped with these power management-capable, mobile, dual-core processors may experience decreased performance or unexpected behavior. Note This problem also applies to x64-based versions of Microsoft Windows Server 2003. However, this article and its associated private hotfix are not intended to resolve timing problems in games and other applications that run on AMD dual-core computers. For more information about performance issues on dual-core computers, click the following article number to view the article in the Microsoft Knowledge Base: 909944 (http://support.microsoft.com/kb/909944/) Game performance may be poor on a Windows XP-based computer that is using a dual-core processor "Kemo

The dual-core fix, was mainly brought out to "fix" XPs API issues. Remember that when XP was released it wasn't made with more than 1 core in mind. This fix was about certain types of software making a compensation for those apps that bypass the XP API for timing by directly using the RDTSC. Apps that rely on RDTSC don't benefit from XPs logic to properly use it's power management.The optimizer helps to correct the negative GPU performance/timings issues/effects that certain applications may experience on dual-core or multiple processor systems.If you do notice excessive stutters or anything very noticeable affecting performance negatively, then install it, as it does fix whatever was wrong before..My advice though, If it's not broken don't start fixing.