:eek:
I wouldn't reccomend it for either but,
If you think it could power 3 x 560Ti's what would make you think it couldn't power a pair of 680's ?

http://guru3d.com/article/geforce-gtx-680-sli-review/4

http://www.guru3d.com/article/geforce-gtx-560-ti-sli-review/14

~90% efficient psu used in reviews
473w AC = ~426w DC = ~ 71% load on a 600 watt psu
449w AC = ~405w DC = ~ 67%

Based on running nothing more strenuous than a game like load for a few hours a day, and not having a fear of surpassing the magical "50% load for optimal efficiency and extended psu life" number during those few hours , either of the configurations in the reviews could run on it ( with the use of adapters )

A third 560 Ti ? No