Don't waste your money on a 2nd HD 7950 with that board. The 2nd PCIe x16 slot is only x4 electrically. It will be a bandwidth bottle neck. Stay with one card.
Short of installing an i7 processor, you are about at max for gaming. Unless you are gaming on 3 hi-rez monitors, I don't see any need to get an HD 7970. The difference wouldn't be noticeable on a single 1920x1080 display.