@hapkido: According to benchmarks here:http://www.videocardbenchmark.net/high_end_gpus.html
The performance difference between 580 and 680 is minimal, for $120 more for the 680. I will evaluate my system once built to determine if I will definitely go with the 580 SLI, but it's what I have in my head at this time. Here's a good article I read about it:http://www.guru3d.com/article/geforce-gtx-580-sli-review/
I don't trust those benchmarks. It has the gtx470 ahead of gtx590 and hd6990. gtx470 is gtx560ti, hd5870, and hd6950 territory.
Offhand, I believe gaming performance for single gpus is 680 > 7970 > 580, 7950. On newegg, 680s aren't in stock, 7970s start at $480, and 580s and 7950s start for $380. If you're set on nvidia then 580s is a good choice (I didn't realize how much they dropped in price). But 7950s is probably a viable option. They use less power (meaning they're cheaper to operate, easier to cool, and you can get by with a smaller psu) and they may overclock better and scale in xfire better. I'm not sure on the latter two things, but it's probably worth looking into.