I'd say if it comes between a 3000+ or a 3.0ghz, the 3.0 ghz because the 3000+ is a pretty badly nerfed chip. Anything above that (3200 or 3400) get AMD. You also have to understand that the Athlon 64's are 64 bit. This doesnt mean anything performace wise now, but in a few years, it may mean very significant things, especially for high performance 3d programs. Regardless, i dont think you will 64 bit versions of software before these processors become obsolete for being a 3d workstation. Essentially, im saying dont worry about 64 bit capability. If you are going to build it from parts, in case you dont know already,
www.pricewatch.com is awesome.