I think 2x 4870X2s are excellent for today's games,there is no need to upgrade them IMO,it seems you are happy with their performance so why upgrade something when it gives you the performance you need
ONE 4870 x2 is plenty for any game out there today. Two of them is more than enough. Quadfire would be insane, not to mention a waste of money, because by the time any game came out that it would help with, I can guarantee you it'd require DirectX 11, which the 4870 doesn't have. You'd just end up throwing away four video cards instead of two.
Just keep your current setup until things come out requiring DX11 in a couple years.