It really depends on what you're playing. If you love Battlefield 3 and the Batman games (both run better on Nvidia) then you will be happier with the GTX 670. If, however, you don't play just those three games, then get the 7970 or 7970 Ghz Edition like lt_dan_zsu and Nanzer said.
To throw a little fuel on the fire, a little pricing comparison:
What amazes me is the power consumption of the AMD cards. The 7970 Ghz Edi uses 10 less watts at idle than the 670 but almost 80 watts more at full load. Nvidia is doing some great things keeping the power down. Now if they would just fix their memory bandwidth on the mid range cards (I'm looking at you 650 Ti & 660 Ti) they would be so much better.