I agree with theonerm2, I don't think you would be getting a bottleneck with dual 580's, maybe with dual 780's but not 580's. Even if you had a slight bottleneck the OC would resolve that. If you're still worried, go ahead an OC the NB as well and then you'll have no bottleneck even if you had something like a 780 in there.
Your 1.5gb ram on each card may limit you when it comes to super intensive games at 2560 res and up but you should be okay for even games like Crysis 3 and BF4 if running at 1080 (possibly not ultra preset as that DEMANDS RAM).