I think the 460 cards and the 68xx cards swap the lead on a few benchmarks though the 6870 generally wins at default clocks.
Sorta, on Anandtech, at stock the 460 1G beats the 6870 in only 1 out of 11 tests and swaps leads depending on resolution in 1 test. It beats the 6850 in 4 tests, swaps in 3 tests, and loses in the 4 tests out of 11 tests. The 460 OC swaps leads with the 6870, however, at the cost of much larger power consumption.
Which probably goes a long ways to explaining the pricing on Newegg.
GTX 460 1 GB - 180+ USD
6850 - 180+ USD
6870 - 220+ USD
And if you talk overclocked. Then OC'd 6850 can sometimes hit 6870 performance levels and over just as OC'd 460's can.
If price and performance were all there was to it, the 6850 and 460 would be evenly matched, but since efficiency is also part of the OP, that gives the nod to the 6850. Same idle power, but MUCH lower load power consumption.
Personally, however, if I were to buy one of the two, I'd go with the Nvidia GTX 460 1 GB just because I detest the naming reshuffle with the 68xx line.
But, since this vote wasn't about personal preference, 6850 is the best price/performance/power efficiency combination card of the current generation.
Regards,
SB