Well I took the plunge, got a Diamond 6870 at the local Frys store. So far pretty happy. I checked out places where my previous fps was dropping to sub 10 fps and now they are above 35fps (check out the place just down from K3 overlooking the valley below Dalaran).
So I suppose % wise I am getting a huge jump (300% on the above example). As for boxing it seems a little smoother too and I am now able to have shadows enabled on my main screen, whereas before the upgrade it killed my fps.
Excited now for Cataclysm
Thanks all for the input.