You will get a better bang for your buck going with the GTX 460. With a decent overclock it can even perform with the 5850 in some cases. 2 x GTX 460's in SLI can even outperform the GTX 480 and HD5870.
I agree, performance wise their is really nothing that can compare with the HD5870 right now except for the GTX 480 and it's heat and power requirements make it unattractive. But looking at sadain's PSU and the rest of his build, it makes the HD5870 an unrealistic option (unless they're willing to upgrade the PSU as well). That's why I suggested the GTX 460.