The 6850 was faster last time Toms hardware tested them against each other (although the difference is only superficial in some DX11 titles, such as a .2 framerate difference in Battlefield 3), however, it is also reported that the latest catalyst drivers have improved performance (In some cases, greatly, such as Crysis 2 DX9 performance, the department the 7770 was lagging behind).
There is no noticable difference in performance between PCI 2.0 and PCI 3.0. PCIe 3.0 allows for faster graphics cards then PCIe 2.0, but at this point, no card has reached the theoretical bottleneck of PCIe 2.0 anyway.
Personally, I'd go with the 7770 (I was making the same consideration a few weeks back), as the 7770 draws less power (80wats as opposed to 150 wats or so) - which is important to me personally due to my PSU. If PSU is not a problem, I'd go with whichever one is currently offered by the brand of your choice (saphire is a good brand) and whichever one is cheaper - they are being sold at identical prices in the Netherlands.
Really depends on your budget and power supply. Neither of those cards is super fast, but the 6850 definitely has an edge. However, in your shoes I'd try (as others have suggested) to get a 6870 instead; it's available for $150 post-rebate on newegg and is about 20% faster than the 6850. Even if it's a few more dollars than you intended, it will last you a lot longer than those other cards, making it a better buy.
It's unfortunate that AMD has not really filled in the gap between the 7770 and the 7850 yet--from about $150 to $230. To my mind, that's always a sweet spot for a graphics card, but at the moment it's being filled by older hardware. Right now it's the 6850 and 6870 (and sort of, the 6950), but it's really dominated by nvidia with the still-available GTX 560 and 560 Ti. For you, though, the 6870 is a better buy than the 560, and the 560 Ti is out of your budget range.