A little caution is advised on very cheap PSUs. I saw some at a fair and was able to take a close look at them.
I saw one marked '500W' for about 29.99 which immediately caught my interest (especially as I'd just paid about 79.99 for a 430W PSU).
It had high-speed sleeve-bearing fans, tiny heatsinks and tiny transformers internally. The main reservoir capacitors were about half the size that I was used to seeing. The power cables were very short, and were a mess because the individual wires were all different lengths.
Best of all, however, were the specifications on the box - I added them up. 3.3 V + 5 V combined power = 170 W. Total power = 295 W.
I'm sure that it would have no difficulty at all in powering an Athlon 64 or Pentium 4 based PC (even one with high-performance gaming graphics and RAID), but you'd have to be supicious as to how long it would last. A high-end PC may need up to 180 W of clean power, but with the PSU sucking hot air directly off the CPU, I'd hate to think how hot things would get inside that PSU. Of course, that assumes that the fan keeps going - sleeve bearings are a disaster for high-speeds and high temperatures.
Don't forget some of the other advantages of high-quality PSUs:
- the newest active PFC PSUs have significantly improved electrical efficiency, and will save you money on your power bill (up to $2/month).
- better quality connectors can significantly improve voltages at the motherboard while keeping temperatures low (I've seen a few mobos with melted molex connectors due to low-quality PSU connectors);
- improved protection: cheap PSUs will usually just try to shut down in the event of a fault. Better PSUs have an over-voltage 'crowbar' which actually absorbs the surge protecting the rest of your comp, as well as guaranteeing that the PSU shuts down.