I've currently got a 5870.
Would I see much improvement by getting 2 of these for 6950CF as opposed to buying a 2nd 5870 and going to 5870CF?
I would say get another 5870. Yeah 6950cf would out perform 5870cf, but not by much. If you want the little extra performance go for the 6950's. If you want to save yourself £200 yet still have very impressive performance, go for another 5870.