Hey there, you're mobo is fine and dandy for any PCIe graphics card and it's SLI compatible (thus single ATI cards will work). Where you run into thorny compatibility issues is with SLI and Crossfire. As I said, it will work with SLI, but not Crossfire (no mobo exists that supports both). I believe one 7950 will out pace two 7600's, plus by buying a single card you'll avoid the issues a few (not many) games have with multi-GPU technology. It'll generate less heat in your case too.
All GeForce 6 series and up cards are SLI capable, and X800 and all X1000 series ATI cards are Crossfire capable. SLI simply links the two cards by an internal bridge, Crossfire requires an external dongle and a special Crossfire version card to accommodate it (except for the 1950 Pro, which uses a bridge like SLI). SLI requires two cards of the same model, Crossfire only requires two of the same series (i.e. X1800 Crossfire will work will X1800XL and X1800XT, but 7950 will only work with another 7950).
Edit: One more thing, SLI/Crossfire tend to shine when high resolution is used and all the eye candy (HDR, AA, AF) is turned on.
you could get 2 76gt,s for around 200$ or so.
and a 79gs single card for almost if not cheaper and the performance
is about the same. if you are playing at say 12x10 or less
i dout youll see any difference.
you could turn up the settings a little higher. with sli.