Take a look at the 6800GT before you buy. It's the same as a 6800 Ultra but clocked a bit slower because it uses a smaller heatsink and quieter fan. Some people with good case cooling are able to run their 6800GTs at Ultra speeds, but of course you might cook your card for what turns out to be only a moderate increase in speed.
The 6800GT blows away the ATI cards sold at that price point ($350-$400), but you should check out benchmarks that use the games you play. In particular, farcry seems to have some problems right now with nvidia cards and the patch to fix it has been delayed for some reason, while a dozen nvidia beta driver updates haven't fixed it either.
For doom3, nvidia cards rock. That's pretty much undisputed. For everything else, you really gotta look at some benchmarks you care about.
