You can't really put a GPU side to side and compare specs, like cars or something.
It depends on all of the other components of a PC.
Hence what maxalge said, in the real world, FPS shows. If you have a rig with a CPU that does not bottleneck the GPU and put one card in, test it, then put another card in and test that one, and the FPS is higher overall, then one card is better than the other.
Of course, that's considering you are comparing the same tier cards (as in not a low budget card with a mid or high tier one).
But if you have got a card, and a factory overclocked version of it, then yes, you can compare the clock and memory speeds.