If my math isn't off, the GTX 460-1GB should be about 13% slower than a GTX 470 at stock.
The GTX 560 w/384 CUDA Cores @ 820Mhz should be that much faster than a stock GTX 470.
I haven't come across any 6850s for $149. The cheapest I've seen are $169(After MIR) and of course two mainstream cards v. one performance card, the mainstream pair will beat the solo card as is the case when you compare two cards of any given segment to any one card in the higher tiered segment.(Ex: Two GTX 560s will beat a single GTX 580..etc.)
And I got my ASUS 6850 BNIB/Sealed on Ebay for $150 shipped. It was a great deal of course but they're easy to find for $169-$179 regularly.