I just did this in vista http://support.microsoft.com/kb/935799.
I belive my problem was the power setting in Vista said Preformance in the area to change powersettings but was in fact set to ballance in the registry and was chocking my cards power. After applying this fix and setting the reg to preformance my FPS jumped from 6-15 to 45-50.There are probably a few more teaks out there but this has made a huge imporvement. Anyway its explained on the website and use it at your own risk, regedits can kill your machine.

edit a second fix for some with multiple cards maybe:
http://support.microsoft.com/kb/936710
" When a DirectX 10 application runs on a Windows Vista-based computer that has multiple graphics cards, the computer does not use the secondary graphics card"