I'm more concerned about what your video driver situation is...
The mobo works fine while running onboard.
But when I want to use my 8600 GT 512, it will only run SOME games.
It sounds like you might be trying to get these to 'co-exist'; when you added the card, did you uninstall the 'on-board' drivers completely, before installing the card drivers?
In the BIOS:
On the "Advanced BIOS Features" page:
"UMA Frame Buffer Size" to "Auto"
"Init Display First" to "PEG"
"Surround View" to "Disabled"
On the "MB Intelligent Tweaker(M.I.T.)" page:
"PCIE Clock (MHz)" to "100", not auto - auto sometimes doesn't work properly...
Should I buy an ATI card to fix this problem?
If you want to use the Hybrid Xfire, this is the only option; trying to get an ATI and an nVidia to work together is like putting a snake and a mongoose in a box! If you will be happy with the performance of the nVidia alone, with a bit of fiddling, it should work fine...