You would have to do some Googling to find which games are better with which card. I think in the 4890 vs 275 tests that Nvidia did better in Age of Conan, but ATI did better in the other games (six games, with and without AA on).
Usually there's at most a few FPS difference, but there are some games where the difference is much higher. Again, go to Google. Check out some reviews for the cards, as most use games as benchmarks. You will also find some direct comparisons to help you better judge the results.