I would get an opteron 165 and overclock it, then get the best graphics card you can get. Alot of ppl havn't gone dx10 yet and are taking advantage of the price drops on some of the dx9 cards, but the high-end dx10 cards still give better performance then the best dx9 cards (the 8800gts 640mb version and the 7950gx2 are on pare in some cases).
I don't plan to upgrade to vista until this summer or when a service pack comes out which ever comes first.
CPU: e6600 @ 3.2ghz
RAM: 2GB G.Skill HZ PC6400
Mobo: ASUS P5B Deluxe
GPU: Sapphire HD4870
Soundcard: Creative Labs X-Fi XtremeGamer Fatal1ty
HDDs: 250GB WD + 750GB WD + 2x1TB WD
Monitor: Samsung 226BW
PSU: OCZ GameXStream 600W
Case: Cooler Master Stacker
OS: XP Professional SP3
Mouse/keyboard: Logitech MX518/Saitek Eclipse II
Speakers: Logitech Z-2300