Do you play older games? If so how much older? I'm not exactly sure, but apparently the 7xxx cards still play older games decently, whereas the 6xx does not or cannot. Not exactly sure on all the details though, you would have to google it.
I don't think there are many driver issues ATM for the 6xx right now (i've read of things like vsync studder issue, sometimes the card not clocking up to full speed, but they seem to be "rare"). A new NVIDIA driver should be released soon.
Basically I would still go for the 670 if I were you. It's just got so much more room for "growth" than the 7950.