My motherboard does support dual graphic cards. For example, a minute earlier, I was on Vista with my 3 monitors on and then bam, on 7, I can't get the third one to work!
The weird thing is that on the first boot, all 3 of them were up-and-running. Then, 7 did some "critical" updates and boom, reboot & only two of them are working.
I installed the ATI Catalyst something software and just tried to force recognition, as you suggested, and it didn't work