Two 4890's perform significantly better than a 5850.....the 5870 would be an improvement but at twice the price of adding a 2nd 4890. With fermi allegedly delayed till march, I don't see the 5xx series dropping anytime soon.
2 4870s (1gb) would equal the performance of a 4870x2 which equals a 5870. 2 4890s out performs a gtx295 and thus a 5870. Personally I'd throw in the other 4890 and skip this gen, or atleast wait till 2011 to see where dx11 is heading. If you can sell it and get enough for a 5870 that is fine too, but honestly it performs to close to a 5850 to be worth investing into. You can overclock a 4890 pretty damn far and close the gap between a 4890 and gtx 285.
Edit: 2x 5770s is also another option if you are itching for dx11.