Don't skimp by buying the cheapest brand power supply the way you wouldn't on a motherboard or video card. You can always try the cheap ones that come in many cases, but when upgrading a PS, go with a name brand. Fortron, Enermax, Antec, Enlight are all good brands. Sparkle makes a decent and cheap power supply too.
If you are not willing to spend $50+ on one of the nicer models, a basic Fortron 350W can be purchased for under $30. I wouldn't really recommend any cheaper than that.
ABIT IS7, P4 2.6C, 512MB Corsair TwinX PC3200LL, Radeon 9800 Pro, Santa Cruz, TruePower 430watt