The Nivadia 8800gtx liquid cooled is top of the line right now, and if you go SLI, you're really spending the bucks..somewhere around 1000 to 1200 bucks.
I'm running the 8600GT, 256mb SLI, with a duel core ADM processor, 2GB ram for just under a grand. And this from Alienware!
You can spend as much or as little as you want, but I think that if you're on a budget, as most of us are, tthe key is to build something that can be upgraded as money permits.