I am looking to upgrade my old athlon xp based mobo's PSU. I know little on the matter, so I need to know what would be a good, compatible PSU that is at least 400watts.
If you're looking for high quality and stability around $50, consider this: Antec Earthwatts EA430
The entire Earthwatts lineup is Top-Notch. I own the 430 and 750. There's only one minor detail - the EA430 may not have quite enough umph on the 3.3v and 5v rails for an older system like yours... Hopefully you can get another opinion.