If your current cards don't cut it, I'm certain dual 5870s would.
Fermi is so up in the air right now, I'd not wait for it you have the money for new cards now.
It could be a stellar performer, or it could be the GFFX series all over again.
It could be released in two months, or four- nobody but nVidia's higher ups know it seems.
So, I'd just get the 5870s. If Fermi comes out and proves to be the best thing since sliced bread- sell the 5870s and get a couple "Fermis". If it proves to be a flop then you're still set with the two 5870s.
That's my opinion.