I agree with what is said above. The two best choices are 3dmark11, or Unigine Heaven. I use both, but mainly use Heaven. The free version of 3dmark11 only allows you to use the performance preset, while the extreme presets are locked (you have to buy the program to unlock it). Unigine heaven is completely free, and unlike the free version of 3dmark you can stress the GPU to the max. It has many different options as far as settings go. If you apply some heavy tessellation at max settings you can really stress a card nicely.
I recommend you try both, as they are both great benchmarks, however my preference if Unigine heaven.
Hope this helps!