Depends on your resolution and also depends on what you consider "just fine"
@ 1920x1080, a 6870 should run it fine with a couple of decreased settings (ubersampling goes without question)
a 6950 at that resolution will do better of course, but you honestly should be just fine with that 6870 as long as you understand you wont be able to max everything out. Hell my 570 can't max everything out now anyways. That's not to say the game will look bad though, Witcher 2 is one of the most beautiful games I have ever played
lol, forgot to add that I'm on a crappy 1280x1024. I'll be switching very soon.
Actually, I'll switch it now. brb