I ran 4x 5830 on a 850w with no problem, Only pulled 708w from the wall.

"No way a 800w will handle 4x5830

5830s take up 188w each about, OC'd they can take up 250w

Even 3 will be cutting it close OC'd but will be alright so as long as you aren't running anything else high power.

For 4 I would get at least a 1000w PSU. I'd go with a 1200w just to have more headroom personally."

188w is the MAX TDP, not the actual power consumption. When you downclock the memory, each of my cards consumes only ~175w, leaving ~150 on my 850w to power everything else (more than enough).