Check for settings like Vsync, and forced AA/AF.
Also check to make sure you are running a proper driver, and the latest one at that.
On that card, you should have NO trouble running trem.
Also keep in mind however, that trem is based on the q3 engine, which doesn't work solely off of your GPU. It is actually more CPU dependant.
So, if you have a crappy CPU but a blazing video card, you might still have some issues.
-Y