So to calculate the mean and get a rough estimation, I've added up the fps difference between the two card comparisons, selecting the resolutions closest to the one I use, then divided that answer by the amount of sums I added up.
Make sense? Are there massive flaws in my massively flawed logic!? The answer I got sounds about right, and if my calculations are correct, then I think I'll invest. A second opinion from some braniac would be nice though.