I prefer a single powerful card, better than SLI/Crossfire. And you will need more power to SLI/Crossfire with a decent PSU.
The powersupply is not a problem. I have been looking forward to crossfiring for a long time now and again I was wondering if the 7950 would work with the 280x. also would I be better off with two gtx 760's?
Yes indeed, SLI works better than Crossfire. I would go for GTX 760 SLI.