Video card is your problem. I'm also assuming that since it only has 64 MB of its own memory, it's stealing 64-192 MB from system RAM via Turbocache (many of Nvidia's low-end PCI-e cards do that).

Actually, if you lower your settings sufficiently, you can hit 30 fps and above. I'm serious. In fact, I recommend running at low settings because on lowest, it looks just as good as the Xbox version, and on high, the difference is minimal but with a serious performance drop. 1024x768 or 800x600 on lowest should do you fine - the game will still look good and the framerates will improve.