I have one rig with 460 and another with 6850. Graphicswise, they perform about the same, although the rig with the 6850 is being held back more by the CPU, so I run the mod with higher settings to shift more of the burden to the GPU. It runs 1920x1080 on high settings with acceptable frame rates.
The problem is overclocking. 6850 just doesn't clock well. The 460 clocks great. For the same ~$150, you can buy a 6850, or a 460 clocked at 850mhz. A bit faster than even the 6870 at those clocks. The other option is a stock clocked GTX 460 1GB at $130 after rebate. Clock it up yourself and you're all set. I've never heard of a 460 that won't hit at least 825mhz, and most go much higher.
If you still don't care and want an AMD product, at $150, the 6850 is a nice card and will pair well with your older quad.