IIRC the 6950 is the most powerful out of the group. The 560 might be slightly faster but not noticeably, and not worth the price discrepancy.
I have two 6850s in crossfire and they chew through every thing I throw at them except for FXAA in some games. Older forms of AA my set up renders no problem I think this is a Vram issue.
All of these cards should be able to max out WOW, and it really depends on what games you want to play via steam. Steam is just a font end for game distribution. Do you mean you want to play source games on max or just games purchased through steam?
It's too bad you can't wait a year and get a 780 GTX.
The 6950 is by far the most powerful of that bunch, but I'd recommend a 7850 instead. More bang for the buck. The others are all good options at their price point, except for the GTX 550 Ti. 6850 and 7770 are pretty evenly matched, the GTX 560 a little better.