I have been in the process of putting a computer together I only have to get a GPU. I was originally going to get a Twin forza II gtx560 ti but I am have more money than I originally planned for, so I am looking for a better card.
I was going to go with a gTX 570 but I just read the article for the best gpu for each price range and they said the sli/crossfire set up was best.
Currently I have i2600k, p8p67 evo, an ssd, CM 690 case, cm hyper 212+, 1tb hdd, 8gb patriot ddr3 memory, and a 1000W PSU.
A pair of GTX 460s or HD 6870s would probably be the fastest setups under 400 bucks, but your probably better off getting a good single card like the 570 or 6970 so you have the option to add another later on.