PCIE 3.0 is pretty much irrelevant until you get to 3 GPU configurations. I wouldn't worry about putting anything in your current mobo as long as your PSU can support it... (PCIE3.0 cards are backward compatible, you can use a PCIE2.0 slot to run them, and they won't be bottlenecked)
the 7850 is the winner for ~200$, the 7950 is the sensible buy at around 330$ (the 7870 is slightly cheaper and slower, not a big diff between em). If you go beyond that, go for the 670 (400ish) if you don't plan on overclocking; the 7970 can overclock better than the 670 from what I've heard and so could be a better value if you get lucky.
edit: Note I only included the current gen
, you can probably get some good values in last gen stuff as well.