A good SLI certified PSU that is at LEAST 700w and 80+ bronze will be fine. Go for a modular PSU, they rock.
As for your queries on overclocking, the 8350 will overclock well, so long as temps are good. Should be able to get 4.5 with little trouble.
In my experience, overclocking GPUs is rather easy. It's actually my area of expertise. There is no reason not to overclock your 760s. I'd try an extra 100Mhz on the clock, and 100-200 on the memory to start. But you really must overclock both cards the same. Might need to increase voltage a bit, testing will help you find out. Your overclock is good if temp are good, and if you don't notice any weird artifacts or crashing.
As for the RAM, you might be able to pull 1866, or lower timings. It's all done in the BIOS. You might need to increase the DRAM voltage a bit to pull it off.
As for the Mobo, it's grand for overclocking. I'd try to increase the NB clock and probably voltage with it to increase stability and speed.