It pretty much comes down to number of hard drives, processor needs, and video card needs. As I recall, the Athlons over 1 GHz need a 300 watt supply. If you're looking to the future and replacing the board, video card, etc, you may want to look a little higher. I've got a 480 Antec PSU in my rig, but I've also got a GeForce 6800 Ultra, which takes 2 separate power lines...
Make sure you get a high quality one. I'd look at spending closer to $100 than $35. A good PSU will keep your computer happy.
