What kind of power supply do you have? If your power supply is right on the minimum recommended for your Video card, it might not be getting enough power and that might cause it to lag like that. I had to replace my power supply when i got a new vid card a while back cuz it would only display have the screen cuz I had a weak power supply.

You ought to have at least a 350W power supply, anything under that is not a good idea with newer video cards.