A GTX680 is faster than a GTX580 hands down, also cooler, makes less noise and uses less power.
check here AnandTech | Bench - GPU13
I dont think that you can get a 680 near a 690 as the 690 has 2 of the 680's gpus.
I use EVGA Presision on my 680's to overclock but I find the 680 at stock to be more than enough for all games @1080p. If your running a higher res it might be worth it. If you dont want to upgrade, consider another 680sc for SLI. That will be enough grunt for a few years.