overclocking is a risk, high overclocks can be had on 7950's but they are not overclocks you would run 24/7 with good stability. take the 670 if you can afford the extra. otherwise the 7950 isnt a bad card. if you are looking to co crossfire or sli in future Nvidia's SLI is much more reliable and better supported
Are there any games that you play that you think would need the extra gig of vram? I'd personally go with the 7950 because I top 2 GB of vram, but that's just me. Also looking at the reviews of the 670 a lot of people are complaining about it being loud and having a buzzing sound so keep that in mind.
the extra vram is timeless to have
the gigabyte windforce 3 670 was $329 from mid nov. 2012 to mid dec. 2012, then prices went back up.
i got an msi twin frozr 3 7950 for $289 and it came with crysis and bioshock earlier this month, then the prices went back up.
right now is not a good time for cards, prices have jumped back up.