I was looking all over for a graph like that. Thanks, Kris. I see this isn't quite like it used to be where you had to use two perfectly matched cards. Now all I need is for the price to come down. On newegg the 6870 are still the same price that I paid a year and half ago like we were discussing in your thread.
I believe nowadays you can use two cards with different clock speeds and amounts of memory--however, they revert to the lowest common denominator when crossfired or in SLI.