32bit XP will support up to 4GB of RAM (virtual memory is limited to 2GB and you won’t see a full 4GB in Windows after SP2). There really isn't an advantage (at least at this point) to running a 64bit system unless you need the addressable memory space. Depending on the manufacturer of the components in your PC you will probably run into some driver headaches with a 64bit OS (please note some manufacturers have great 64bit driver support, others don’t). You will probably also run into apps that will not work on a 64bit native system. I know at work a number of our PCB design systems are 64bit and some of our older apps refuse to run on them.
Performance wise I don't think you'll see a noticeable difference in game play. I've worked with both 64bit XP and 64bit Vista and have seen little advantage to the 64bit versions aside from the larger address space. 4GB is a LOT of memory…unless you’re working on HUGE Photoshop projects, multi-layer cad drawings, 3D Design/Animation, or any type of document that is enormous, you won’t be needing that much memory anytime in the near future.
One of the major problems running windows xp x64 is the process threading between 32 and 64 bit applications. A 64 bit function can not call a 32 bit library, and vise versa. One of the only real benefits for x64 is the windows 2003 server kernel, which is Microsoft's most stable kernel to date. That and you are immune to any 32 bit viruses as well. My experience with x64 is as follows:
Thankfully steam provides us with 64 bit binaries of all their games. They really do not run any better what so ever, so thats basically a null issue. Other than that, all the other programs I ran were 32 bit. World of Warcraft did not like running under WOW64 (windows on windows), as well as any other 32 bit program. Also added to that fact was the major issues I had with IE64. So basically I did not see any gain in running x64.
One of the good things you can do however with xp 32 is limit the kernel to the upper portion of your 4 GB system. If you add the 3GB switch in the boot.ini file, it will force windows to use the last 1GB for the kernel and its function calls, and give you the lower portion of your ram for applications. That should speed up the kernel calls a little bit. Happy Hunting!!