It will (probably), BUT it will be not that noticeable IMO. Moreover if you run it at 3.8Ghz+.
Now, Crossfire depends also on the PCIe lanes. If you'r thinking about 790 or 890 MoBos you'll be fine in that aspect. CPU wise, I really don't know if it will be THAT horrendous; my intuition says it won't, but don't have data (on that CPU) to back up that claim 8(
They're the same microarchitecture minus the l3 cache, but the number of cores makes more of a difference than cache. Both are still underpowered if you want to cf 6950. If you want AMD, it mind be better to hold off til bulldozer, otherwise nothing beats intel right now at the high end.
NOTE!!: I heard not everyone is successful with it. If you choose this path, you'll be doing it at your own risk, and you should view unlocking cores only as a bonus. Use the latest BIOS version to maximise your chances.