Alright...just a few things.
1. Wait a few months so the R520 gets out. X850s will drop in price.
2. Make that mobo Crossfire ready. It's always handy to have two PCI-e slots, and if rumors have it that SLI works on those boards (Anandtech Forums), then they're more worthwhile.
3. Get no less than a 3800+ X2
4. 64 bit XP is shit. No good drivers out. Get regular 32 bit now, 64 bit Vista later.