Best & most expensive = 690
Recommended = 2x 7970 non-GHz and overclock them (or 2x 680s which will be around $200 more expensive and almost 690 price range.)
Suggested: Look at the new Geforce Titan, 6GB of video memory and almost 690 performance, but again, same price as 690 if not more.
The main issue with the 690 is the 2GB of memory per GPU. Yes they say 4GB on the box, but that is 2x 2GB and when SLI, you are limited to 2GB.