If you can get a 4890 for under $200, then go for it. Otherwise, it's more worth it to shell out $270 for a 5850.
A 5850 is about 5-10% better than a GTX285. And a GTX285 is about 10-15% better than a 4890.
DX11 won't be implemented for a while, so getting a 4890 will hold you until DX11 matures and the prices of the ATI and Nvidia DX11 cards drop.
(or get a 4870 for about $150)