Actually........ the 670 (particularly the 4gb) typically only benchmarks 4-5% slower than a 680. Is an extra 5% (even 6%) worth an extra $130 price jump? Hardly.
A friend and I did a real world test just to see. I had a single 670 4gb, he had a single 680 4gb. Neither were superclocked.
He only outran me by 1-2 FPS. He lost a bet that day. He swore that he'd outrun my vid card by at least
If you've got the pocket for it by all means, go big or go home. Me personnaly, if I did it again today, I'd still run 2 of these: http://www.amazon.com/dp/B00966IRL8/?tag=pcpapi-20