Nothing is wrong with getting a used 670 also, I sold my friend my gtx 670 for $150 and he now uses it everyday for gaming, it's a good card, a little faster than 760 also (even when both overclocked) and I'm sure you can easily find someone selling it since it came out a while ago and a lot of gamers update GPU about every two years. They did squeeze about 10% performance increase from 680 to 770 so I would say the 770 is 15%-20% faster than 670 as 680 is 5%-10% faster than 670.