If you buy a system now (especially an AMD) it's a no brainer to get a 64-bit CPU. They're not all that much more expensive, it keeps you 64bit compliant for the future, and the 64bit processors (even running 32bit Windows) are faster than their 32bit counterparts of the same clock speed).
HOWEVER you may not want to rush into going with WinXP 64bit operating system just yet though (and yes you do need 64bit WinXP installed to make use of 64-bit applications).
IMO (for what that worth) 64 bit gaming won't be a reality until after the end of this year. Some existing games have 64 bit ports like FarCry but they're few and in between.
Meanwhile, since 64bit is still new and drivers aren't fully optimized yet you will actually see some performance loss in 32bit games as evidenced in this article here:
http://www.simhq.com/_technology/technology_043a.html
Me personally I would wait until this Xmas when we'll probably see a bunch of REAL 64bit games (designed for it from the ground up for 64bit) and hopefully Service Pack 1 will be out for WinXP 64bit.
If you do upgrade to 64bit be VERY sure you have 64bit drivers available for all your hardware. Plenty of people are having problems just find sound drivers. This will get better over time, although support for some older hardware may never be available.