Based on previous generation gains, I'm thinking a 30-35% gain. GTX670 was ~40% faster than GTX570. GTX570 was ~25% faster than GTX470. And GTX470 was ~30% faster than GTX275. So it could be as little as 25% (or less!) or it could be much more. I'd say though that even GTX670 will give a major performance boost over CF 5770s, so a GTX770 should be a huge upgrade.