I would say there is no reason to do a CPU/mobo upgrade at all. An i5 760, especially an overclocked one like yours is still quite good. Getting a Bulldozer or Sandy Bridge system probably wouldn't yield any real performance boost in any game, not even in the really CPU heavy titles, not unless you are comparing the CPU's performance when using a really high end dual GPU setup. Right now, your performance bottleneck is definitely the 6870, not the i5 760. If you really want better gaming performance, either add another 6870, or look at a better GPU.
If you do want to just upgrade your GPU without taking the Crossfire option, you may want to wait until next year for the next generation of cards. From a 6870, the only really worthwhile single GPU upgrade is the GTX 580, and it is horribly overpriced for the performance it gives.