May I suggest you do the i7 build and pop the 8800 in as a stop-gap? It is by no means a weak card and you can then make a decision on DX11 hardware when it arrives-While still playing with that nice, shiny i7
Or you could do the above, and put a good but cheap card in the current rig and sell it as a going concern, with a HD4870 it's still going to a pretty good gaming system.