You could get away with it just fine stock, but if you want to OC at all, the headroom that you have will disappear very quickly. The 3930k can go from its 130w TDP to 300w pretty quickly, and even 400-500w if you push it. At that point you'd be struggling just to run your SLI 780s, say nothing of overclocking them to where they'll really gallop.
So that's all up to you. The HX 750 is a great choice for a stock setup or mild overclocking.