If you only have $400 total then there is a radeon 7970 ghz edition for $409 on Newegg or a GTX 770 for $399. Either would be great in that price range. Some games will run better on a radeon 7970 ghz and some run better on a GTX 770 but they are very similar in overall performance.
If you want to spend $400 per card and do two of them ($800 total) then I'd avoid the Radeons and do two GTX 770 cards. Nvidia SLI is more consistent than AMD CrossFire.