@50bmg: DX10 and 11 is Win 7/Vista only and you do not mention your OS.
The C2D is going to hold back dual 4870's a little, but not too much-a little overclock will go a long way to addressing this 'problem' and a cheap cooler (coolermaster Hyper 212 plus) is more than enough to keep the temperatures down on such a chip.
You're not missing much without DX11 apart from a huge drop in framerates (from DX9) and I doubt the developers will stop supporting DX9/10 soon so the second card is not likely to become a doorstop very quickly.
Me-I'd grab the second card in a heartbeat, but make sure the PSU can handle it.
Adding a second 4870 to my system, even though they are 512mb cards, was pure awesome. The C2Q offer very similar performance clock-per-clock (even slightly better by some peoples opinions) when compared to Phenom IIs, so our experiences should be similar. I personally decided to wait another generation for a DX11 card, seeing as how neither NVidia nor ATI gives spectacular tessellation performance yet, and that seems to be the DX11 feature I am most interested in, personally.