No. At higher clocks the 4850s in CF will spank the crap out of stock CF 4870, but the 4870 can OC more because of the higher memory clocks. Trust me I have CF 4870 and there is not much more performance in games, but in everything else it is really nice. It was really nice before when I only had one xD.
My point is that if you're looking out for that next big game 2x4850 will do more than enough. 2x4850 beats a GTX280; my single 4870 maxed out all my games save Crysis which I don't really play. Newer games from trusted developers, e.g. Starcraft 2, Left 4 Dead, Far Cry 2 and expansions to MMOs like Lich King aren't going to get close to pissing on a 4870. Bang for the buck 2x4850 are amazing and in practice they are amazing. Remember they have the SAME gpu as the 4870.