Id say its hard to tell but assuming that the x8 slot was completely bottlenecking the 5870 then any more graphical horse power would be unused, as expected.
Now the hard part is if (Fermi) GF 470/480 will be better than the 5870, if so then the difference in graphical power will not be used. So how much % is that? We wont know till they are tested, which mostlikely will happen within a few weeks of the cards coming out Id say.
Edit: now that I think of it if you could afford two of the highest end Fermi cards one might think why you did not get an LGA 1366 motherboard with dual x16 slots. Or whether your LGA 1156 CPU wont bottleneck both of the cards.