The first item to consider is your CPU. Some of today's high end chips want 100 watts all by themselves. If, for example, you intended on salvaging an older case with a 250 watt power supply, and required to base your unit on a Pentium 4,. You might be able to save that case, but the power supply will have to be change.

The cause is all the other items drinking electricity from that same little well. Your motherboard will simply require 15 to 30 watts, creation it one of the more economical components. But a graphics card, of 128MB or more, will want another 100 watts. Add a hard drive, at 30 watts, and you've used up your capacity. There will be nothing left for RAM, a CD/DVD, or any PCI cards you may want.

RAM has to have seven watts per 128MB. A CD/DVD Rewritable drive takes an extra 30 watts. Those PCI cards are five watts each. Add 20 watts for a floppy drive, and additional case fans.

The total for all this comes out to around 380 watts. A 400 watt power supply hardly covers it. Actually you would be well advised to acquire a 450 watt or larger just to be safe. Not having enough power can mean a fried computer