No noticeable difference in gaming. You don't need a monster CPU for gaming, most games are fine with mid range and even low end CPUs.
As far as the differences, Ivy Bridge is about 5%-8% faster clock for flock than Sandy Bridge. It also enabled PCIe 3.0. No, PCIe 3.0 won't give you better performance than 2.1. Less than a 1% difference in performance, the cards just don't need the bandwidth yet.