Witch is faster 9800gx2Quad SLI or a single 5870

Not really, the only reason to updrade would be for the DX11/eye-finity and or lower power consumption. And the fact that the 5870 is way more future-proof.

Dual GX2's is like having dual 285's give or take, If you want to see an improvement over what you have right now you will need a 5970.