6850 I'd get, the 460gtx is WAY faster than the 550ti, you've got to look at the shaders, the 460 has 336shaders, the 550ti has 192, though also significantly higher clock speed.
Even then the 550ti will have very limited overclocking headroom as its essentially near its maximum already, its highly likely the 460gtx will clock pretty damn high. I wouldn't be surprised if the 460/6850 aren't far off 60-70% faster.
AMD/Nvidia shaders aren't comparible but the 6850 should be a bit faster at stock than the 460gtx, and both the 6850/460gtx should overclock a decent amount.
Either are a MUCH better buy than the 550ti, WAY WAY better.
http://www.anandtech.com/show/4221/nvidias-gtx-550-ti-coming-up-short-at-150/5
pretty much sums it up, the overclock is even less than I thought, 980Mhz is less than 10% over stock speed so add 10% to those scores and its what, not very much, its likely you can get a easy 10-20% overclock on the 6850/460gtx.
Noise and power, the 6850 has a bit of an advantage but on lower end cards, both should be plenty quiet. If you have a preference between Nvidia/AMD go with either. I'd probably recommend the 6850 over the 460gtx..... just DON'T get the 550ti.
