I want to connect a 400w Pentium 4 ATX power supply to my AMD Sepron 2800+ motherboard. Will this work okay?
ATX power supplies will work with any system that adheres to the ATX standard.
However, if you see/have a PSU marked "Pentium 4" and "ATX", then it's probably a PSU from 2000-2004 based on a design from 1995-2000, which was given an ATX12V connector when the ATX12V standard was introduced, without accounting for the greater +12V load demanded by the system.
What I'm trying to convey is, that power supply is probably garbage.
Any modern, decent ATX12V v2.x PSU would do the job. Got a budget?