New PCI versions should be downwards compatible. So it's OK to have a PCI 3.0 card in a PCI 2.0 slot. I say "should" only because within the multitude of models there might be a non-standard one.
Speaking of efficiency, the higher the "tens" digit is, the more powerful the card is within its series. Thus a gtx 5
80 is faster than a gtx 7
50. There's other variables to take into consideration, though, one being the amount and speed of the video card memory. 2 GB of GDDR5 might be enough for AH3 but more is usually better. Also, you're only partially right in assuming that higher clock rates mean better performance. Pages like
http://www.videocardbenchmark.net/high_end_gpus.html will give you a rough idea about each candidate's performance. For further comparison google e.g. for "gtx 560 vs hd6970". There's sites doing such comparisons for almost every card. Read them with caution, though: Especially if the cards are direct rivals, it's more a question of personal preferences and choice of games played than real performance differences! Anyway, they'll help you choose between equal performers if their prices vary a lot, or choose the better one from two equally priced.
