Core 2 uses LGA775, but the reason it doesn't work on older boards is because of the voltage regulators aren't able to provide the lower voltage Core 2 requires. 775 isn't dead yet, and Intel hasn't made a move to can it anytime soon. I'd still consider it if I were you. Core 2 is 32-bit/64-bit ready just like the Athlon 64's are, not to mention the E6400 is less than half the price of the FX-62 and performs just as well. Right now, Intel's CPU's are better than what AMD offers, and that's something we haven't been able to say for a few years now. If you want the best, you're cutting yourself short by not considering Core 2.

Secondly, XP 64-bit is a bad choice due to 3rd party support. Not all programs are supported, let alone drivers and games. 64-bit does have it's advantages, but I don't think we're likely to see a real push for 64-bit in droves until Vista hits, and those drivers will be for Vista.

One thing you may want to be careful of is the motherboard. If you're looking into a multi-GPU setup, realise that you'll need an Radeon Xpress chipset or Intel 975/P965 chipset for Crossfire, and an nForce 4/500 board for SLI. It's nothing to do with hardware, but the platform on which SLI and Crossfire will work is limited through drivers.