Does it really matter ? Both top of the lines crank out avout the same FPS, depending on the benchmarks an ATI card outperforms an nVidia with 2 or 3 FPS and another benchmark shows the other way around. Meaning; both cards are fast, some games like nVidia, some like ATI, the difference is minor.
Not helpfull huh ?
It may also depends on the mobo tho...