As I said, it works fine with my other card (ATI HD4850).
There is no option to explicitly disable onboard graphics. The motherboard is supposed to do it automatically when a discrete card is installed (like it did with 4850).
This is the motherboard: http://www.asrock.com/mb/overview.as...=AOD790GX/128M
Anyway I sent the card for RMA, couldn't find any solution. I hope they find a problem and change it, else I'm in trouble.