There is no thing as a HD 7900, it could be a 7990, 7970 or a 7950. But any of those cards should run thids just fine. It certainly seems like a driver issue with AMD. I'm running a GTX 660TI and I don't have any problems.
64MB of video memory? I feel bad for you man... No, system memory will not work unless you're running integrated graphics, but those aren't too great anyway...
Eh, it's all right. The demo runs reasonable well, though there is a bit of lag...hopefully the full game will run a bit better.