If game performance or value is the most important factor, in my opinion AMD is the only way to go. Check out these
Half-Life 2 benchmarks. Even the Athlon64 3200+ beats every Intel CPU in the test.
At
monarchcomputer.com, you can get an A64 3200+, a good nForce4 Ultra motherboard, plus a free copy of Farcry and HL2 for $326, and you can probably get even lower if you shop around. (I love Monarch because they're in Georgia so I don't get screwed with CA sales tax and they have free shipping on a lot of stuff, plus some of the lowest prices around.) You can also upgrade to a dual core processor later since it's socket 939, and future-proof for 64-bit Windows and apps.
Also, the 6800 Ultra is more DirectX 9 compatible than any ATI card so that's not an issue. If you spend a little more on an nForce 4 motherboard, you can get one that supports SLI, and drop in another card later for a nice performance boost. That is, if you get the PCI Express version of the card, which is more expensive, more money more money, yeah, it's not the end all be all of performance, and I personally did go the ATI route instead of worrying about that. Right now it seems that nVidia vs. ATI is more of a personal preference thing as they're pretty even at the same price points. I happened to get a good deal on my card so I went with it and I'm happy with it. Nvidia is making an Intel chipset that supports SLI but I don't think it's a shipping product yet.