Keep in mind that due to the complete lack of 64bit support in Windows XP and audio apps for Windows I suggest you to stick with 32bit CPUs for now since you cannot exploit the CPU's capabilities like more than 2GB RAM support, 64 bit programs that run much faster etc.
(it's projected that it will take at least 1-2 more years for the Windows OS/apps fully supporting 64 bit, there are lots of technological barriers (see the pcworld link I posted on the forum some time ago).

So for now happily stick to 32bit CPUs and wait for the 64bit CPU coming down in price.
At least if you plan to use Windows, with Linux the story is totally different, see my old post

cheers,
Benno
http://www.linuxsampler.org