It would be a step up about 50-60%.
For example 8800GT=9800GT=X2=9800GX2 which is about 50% slower than the 570.
Ummm, I don't know if those numbers would hold up.... I upgraded to a GTX 570 from two 8800GT's in SLI and in 3DMark06 I only got a 5.1% increase from 15,819 to 16,629. I know there are new features present but talk about being bummed out.... $300+ was a waste for me in playing Battlefield Bad Company 2....