Windows 64-bit is fairly worthless to anyone who doesn't use applications like Visual Studio 2005/2008, Maya, Softimage, 3D Studio Max, Photoshop, etc.
And more often you will see a bigger performance increase from a multi-core 32-bit processor (like Core 2 Duo) than a native 64-bit application. It really is only worth the upgrade if you're an artist needing that extra precision without the speed loss.
In which case most of the time Apple Mac is usually the better route.
You'll find that more 32-bit Processors, like Core 2 Duo now have 64-bit extensions. These will be used even on 32-bit OS, but something interesting to know is both systems for Windows install to different directories.
Win32 - $(Windows)\System32
Win64 - $(Windows)\SystemWow64
It is entirely possible with Windows XP and Vista to have both versions installed on the same machine, in the same Windows directory (although difficult to get it to do this) so that you can multi-boot the same OS in either 32-bit (for compatibility) or 64-bit (for enhanced precision & performance with 64-bit apps)
64-bit Windows is about as good at running 32-bit applications as 32-bit Windows is at running 8/16-bit applications. Often it's quite a hit'n'miss affair; so I wouldn't recommend it for regular use.
