This is how I did it:
I wasn't exactly sure what to do in my BIOS, so I resorted to changing a setting one at a time (I'm such a noob), and seeing if it fixed the problem. After a few tries, I changed AGP aperture from 128mb to 256mb. This fixed my problem. I do not understand how that works, but I'm not gonna question it for now.
Thanks to all those that pointed me in the right direction (BIOS). =)