I'd get an Evga GTX 260 for 220usd on Newegg.
OC it to 660-700 on the core. Evga supports that under their lifetime warranty as well as HSF replacement. The also have a nice tool available to do just that. It works well with the latest drivers and under Vista x64.
If you decide to go X58 and i7 down the line you could grab another for SLi.
If you're looking for the absolute fastest single GPU you can get, buy the GTX 280. I feel the GTX 260 or 260 216SP are better deals though.