There's a lot of misinformation floating around the web about 64bit. The cross-compatibility is nowhere near what it's cracked up to be.
First of all, just because you have 8GB of RAM doesn't mean any single 32bit application you have, is actually going to use more than 4GB of RAM.
Second of all, Windows XP x64 is basically a stripped-down version of Windows Server 2003 64bit more than it is "an upgraded 32bit XP". What difference does that make? Well.. any application you want to use that very deeply interacts with the operating system, isn't going to work unless it was specifically designed for Windows Server or specifically designed for 64bit. You've got next-to-no firewalls to choose from (though this isn't much of a problem with no worms targeting Windows XP x64), then there's Symantec Antivirus (which is by no means bad antivirus - but it's supposed to cost you dearly). Many 32bit programs with shell-extensions are screwed on XP x64.
Plus like I said before, I'm going to ditch XP x64 mainly because you can't get more than 10 simultaneous network/internet connections out of it.