I vote 5850 if you can find one. Granted it costs more than (2) 4850's but doesn't require as much power, would run cooler, has DX11, and best of all is a single card.
Out of your two choices though, a pair of 4850's would out perform a single gtx275.
If you are on the cheap, the (2)4850's are the best bang for the buck.