Well, for one thing, the Athlon64 is 64bit and 32bit. So it is supported by WindowsXP and any other operating system that supports 32bit x86. The 64bit extentions are not supported though.
It really depends on how your cash flow is and what kind of deal you can get on a processor. If you can get a 3.0 p4 instead of a 3.2 p4 with a price difference of about the cost of a 256mb stick of ram, then I'd go with more ram and a slightly slower processor. Otherwise, go with the fastest processor and motherboard now, and upgrade the ram to 768 or 1 gig later.
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