What they said. I generally prefer nVidia because I think the Physx is going to become a bigger advantage as more games are optimized for it. But that is a great price on the 6850 linked above, with no good GTX 460s close, at least at newegg.
Did you confirm you PSU has the power and connectors for whichever one you get?
At 1440x900 you don't even need the GTX 460 or ATI 6850. Both are more than what's needed. As was already stated though, both tend to run neck and neck in many benchmark/reviews online. All depends on which camp you prefer (nVidia vs ATI) or what particular games you play.
ATI has the advantage of being able to do Eyefinity with 3 monitors and only 1 GPU. nVidia needs two GPUs to do 3 monitors; but they have PhysX which is kinda cool. Granted to use PhysX you really need 2 GPUs to keep your FPS up while running high levels of graphics + PhysX.
I lean toward nVidia though. And the MSI GTX 460 HAWK edition is hard to beat.
Out of those two, I'd pick the MSI GTX 460 Hawk, because it has a very good cooler that should allow for high overclocks, and the GTX 460, being a great overclocker would be able to match the GTX 470 and HD 6870 when overclocked. The HD 6850 is also a good overclocker, but the GTX 460 has better overclocking scaling( ie. increase in performance).The price is also good for the Hawk, considering that is a card with an Aftermarket cooler.
Yeah, the Hawk is a very nice card and would be my recommendation out of the cards you listed, especially if you are going to overclock.
That said either card is overkill for your current resolution. I would recommend scaling back to am HD5770 or at least the 768mb GTX 460 unless you plan to upgrade the monitor.