I'd assume it would be the game if everything else works perfectly.
The default drivers should be fine since it comes from the people that made the card but yeah again, I said that 3rd party drivers can be better in some circumstances though i don't know if the DNA drivers will be up to the version ATI is with there catalyst which offers superior performance in new cards like X800's, like 10fps more in games and stuff.
So i'd watch for that