It all depends on how much you pay for electricity, really. From what I know the 6850 hashes around 250 MH/s, which isn’t much, especially considering the recent moves of the market, so it is going to be a tossup at that speed right now and almost irrelevant once the ASIC’s come out on the scene.
While I’m all into Bitcoin, this may be a case at some point where you would be better off mining Litecoin and exchanging that for Bitcoin.