As usual, my recommendations for multi-GPU setups are the 2 highest-end GPU (GTX670/680 or HD7950/7970). In your case, the HD7950 is probably as high as you can get right now. There's quite a few 7950 around $300 and some of them are very good ones like Gigabyte's Windforce, Asus DCUII, MSI's Twin Frozr. http://ca.pcpartpicker.com/parts/video-card/#c=71&sort=a5
There's not much other gaming features. Current AMD cards overclocks better, Nvidia GTX600 cards have TXAA and Adaptive V-Sync. That's pretty much about it.