Huge article in the May Sound on Sound about this very topic
Are PC Musicians ready for Windows x64?The 64-bit Question
With advantages such as more efficient processing and the ability to address up to a Terabyte of RAM, Windows XP Professional x64 has a lot to offer PC-based musicians ? in theory. We put the fully 64-bit system idea to the test.
I read it while at the newstand and it had some very good info in it and would recommend the read if you have not already.
One major point made was that most software available now that we use would not take advantage of most 64bit features...However as a counterpoint you could use some of the new motherboards that support more RAM than you would know what to do with which is always a good thing. You can also just buy this article here
http://www.soundonsound.com/sos/may0...ician_0506.htm