If the CPU is the brain, the GPU is the muscle, the case is the skin, then the PSU is the blood. Don't skimp on the power supply. Get a lower power PSU from a better manufacturer. It can make all the difference, believe me.
Also get 80+ gold certified at least. That'll insure you have a good efficiency and aren't wasting watts at the wall.
Edit: and whether you have one or multiple rails shouldn't matter if it's a quality PSU