It should be impossible to damage a GPU by over working it.
The only way to kill a gpu is to overheating it (the fans clogs and you dont clean it), your case has very little ventelation)
Overclock it, over clocking and over volting can damage a GPU, but your not doing that so its not an issue.
You should run the highest resolution and highest graphics settings the computer can. If the game stutters or lags, turn the games graphics settings down.