listen if you want lower power high performance no fuss 1080p gaming than get a 670! If you want performance everywhere, have patience and willing to tweak than the 7970Ghz can be the fastest single GPU money can buy right now.
AMD has been putting out drivers that are steadily increasing the performance, Tahiti is a new architecture unlike Kepler which is an evolution of Fermi therefore it may have more headroom.
one question about the 670 can I run mods(syrim/fallout/minecraft) less effectively than the 7970 because of the lower bandwidth...
that is a pretty subjective question and can be used for a bit of troll bait.
you will find that each card has better qualities that the other hasn't in different situations. but if you are going to go through them with a fine tooth comb; it will only lead you and other people to pull their hair out with indecision.
though to answer your question; the only time lower bandwidth comes into play is when 8x MSAA is used. other than that, nothing has been proven.
OK well it turns out that i was banned for a thing i did not do so im on this account now... i got the 670 and everthing is up and running but i think the 8120 is bottlenecking it because im getting 40-60 in bf3 and the gpu usage is at like 60% average use