The 7850 is generally faster than the 6950. WoW may be a rare exception to that. The 6950s are starting to disappear, so if you do want to Crossfire down the road, you may have problems finding a second 6950. I would say get the 7850 unless WoW is literally the only game you play. In that case, just one 6950 will be more than sufficient for you, you don't need Crossfire for WoW.
Your 650 Watt PSU also probably wouldn't be enough to Crossfire 2 6950s, they use too much power for you to do that safely. 2 7850s may be possible on a 650 Watt, but I still don't recommend it. If you want to Crossfire 2 high end cards, get a 750 Watt power supply to be safe.