Here's two builds at different price levels - great value and very fast - but you may want to wait for DX11 cards next month. If there's any advice I can give you, it's build it yourself. If you want us to suggest parts it's helpful to give us an overall budget and what games/settings/resolutions you intend to play with it.
CPU: Phenom II X4 945 ($170)
GPU: Radeon HD 4890 ($180)
Motherboard: AMD 770 ($60)
RAM: 4GB DDR2 ($37)
Optical drive: CD/DVD Burner ($27)
Case with 450W PSU for about $40
Total: $514
CPU: Phenom II X3 720 ($120)
GPU: Radeon HD 4850 OC ($90)
Motherboard: AMD 770 ($60)
RAM: 4GB DDR2 ($37)
Optical drive: CD/DVD Burner ($27)
Case with 450W PSU for about $40
Total: $374
Bottom build will play any game (except Crysis) on High to Max settings at 1600x1200 resolution; Top build will do the same at 1920x1200. Transfer your existing OS to them, or if you can't then get OEM Vista Home Premium for $100 with Win7 upgrade coupon.
