It's not easy to troubleshoot this. Undoubtedly your rig should perform better with OFF.
We should try to devise if it's a hardware or software issue. Do you have an extra disk where you could install XP?
You can also try to install the other graphic card you have and see if it's card related or not. I doubt that this will change anything, but we have to try everything to perceive why your rig is under-performing with OFF.
There is no problem with OFF and AMD processors as far as I know. The same with 5870, I've already tried one with OFF. Concerning Win7, both 32 and 64 versions, still no problem. So it must be something local in your rig or a combination of factors.
Sorry not to be much helpful, but I honestly don't find any reason why OFF doesn't perform well in your machine.