[astyanax]

Any ATX power supply should be fine. You can pick up a 300W or 400W dirt cheap. (General rule, the “heavier” (weight) the supply, the better quality it will be. Difficult to know if you're buying online, of course.)
If you're interested in testing yours (and have a meter) you can disconnect everything (except the AC cord obviously) and jumper pin 14(PS-ON) on the 20-pin header (usually a green wire) to ground/common (any of the blacks, 13 or 15 will work).
Then check for 5 and 12v on the red and yellow wires of the 4-pin connectors (using the center blacks as ground.)
Keep in mind this won't necessarily tell you if it's working with a load, but it's a good initial check.

[2 Kool 2 Be 4-Gotten]

I’ve had an emachines for like 5 years and it’s been a solid performer - had to replace the HD recently but was able to use the restore disk to put the OS back on and works great again. In general the only way they can sell these machines so cheap is to use off the shelf parts, so pretty much anything you can think of is pretty much standard.