Laggy, and says only 3 MB Display Graphics

[lukethedonkey]

I managed to install OS X after some problems, but the maximum resolution I can set is 1024 x 768, and, in "About This Mac" it says I only have 3 MB display graphics memory. I actually have 2 Sapphire R9 290s, which each have 4 GB memory, so what's the problem here, and how can I fix it?

Thanks,