5830 performs equal to a 4890/GX275 but runs cooler and supports DX11.
They are better than the GTX260 (which performs equal to the 4870 or 5770)
The GTX470 performs slightly better than the 5850. The GTX465 performs slightly better than the 5830. So I suspect the GTX460 will perform slightly worse than the 5830. In that case, just get the 5830 - cheaper, runs cooler, and runs faster.