If it costs nothing, then yes upgrade. It is a much better card. Only thing i could possibly think that could matter is if you were playing at higher resolutions (1200p/1440p/1600p) or had a multiple monitor eyefinity setup (w/e the nvidia equiv is). I still think the 680 would perform better though. I'm not an expert on nvidia cards but this should be fairly accurate.
The 680 would perform better. It's been stated as where you would need Sli 580's for a 3D Surround setup you need a single 680 for the job.
The performance of the 680 is very much more superior to that of even 590 only because this isn't really the flagship GK110 that should have been released. Expect something ridiculous from Nvidia next year with the 700 series.