It might be good enough. The 955 can handle a GTX580, so it should do 680 too.
And that overclock wouldn't bottleneck it too. Or as far as I know, SLi or Crossfire would definitely bottleneck it, apart from that, nom it won't be a bottleneck.
I've tried to get the overclock higher using multiple ways, always is unstable anything higher than 3.6, kinda disappointing.