A critical mistake made by a lot of system builders is to focus only on CPUs and GPUs, and not care about things like a quality power supply. I learned this the hard way.
A couple of years ago I built a kick butt system and used a cheap power supply. I was looking at total wattage, because that's what really matters, right? WRONG! The cheap power supply caused voltage spikes, and within a week the spikes ended up killing my motherboard, CPU and video card. My losses BY FAR outweighed the $30 or $40 I saved by going cheap. Please don't make the same mistake!