I don't know that any FPS differences between the two are perceptible in normal gameplay.
IQ, on the other hand, is a very noticeable fact.
Moving from a very low-end combo of two 9600GT cards to the 4870X2, I can now run 80+ fps with all eye-candy on at full 2560x1600 resolution in GRiD (to take one game in the chart). I did not see the game get any smoother than the ~40fps I was getting earlier, but for sure the eye candy was a refreshing change.
Without FRAPS, I would wonder how many could actually see the differences between 60fps and 600fps, and you don't need to drop megabucks on a top-of-the-line card to get 60fps at 1680x on most any game out there. As a matter of fact, if you don't have a 1920x or 2560x monitor, it wouldn't make sense to get the top-end cards anyway, so the discussion is academic.
Benchmarks, after a point, are academic.