I hear from developers that XP's 64 bit video drivers are no where near as polished as Vista's 64 bit drivers.
Personally I'd go for a quad core - cheap I7 might be very nice. Load up anywhere from 4 - 12GB RAM - depending on whether you want to do graphics heavy processing (you don't have to get all your RAM in one acquisition). Personally I'd go for more that 512MB on a video card. If you're going real high end $$$ - go NVidia, if your going mid range ATI.
Last advice - plan your purchase and visit fluidtek for the lowest prices if you're building yourself. Latest edition of AtomicMPC has a good review of motherboards, video cards and CPUs.
http://www.fluidtek.com.au/