750W Corsair should be enough. Get only Corsair (or similar reputed) brand though.

Note that many GPU renderers right now only do CUDA, which is nVIDIA,

Not much at all. The main reason people buy those kinds of RAM is that when you overclock, it gives you more headroom to work with. But RAM is not really the bottleneck these days.

Yes. There is some risk involved but the benefits are more then worth it. There is a dramatic difference in performance between a stock 3GHz CPU and one OC to (say) 4GHz.

Thermalright is pretty much the gold standard here. Don't bother with water cooling unless you live in a really really hot area though.