Going by what we've seen, the 4850 seems to be about the same speed as the 9800 GTX, the 4870 is supposed to be about 1.3x that, when Crossfire works, it scales by about 70% on average in dual card situations? And the GTX280 is said to be anywhere from 1.5-1.8x a 9800 GTX (correct me if I'm wrong, I've seen all sorts of numbers for it!), I think two 4850's vs. a GTX 280 would be a close call, two 4870's would probably edge it out.