If the motherboard has a 4-pin 12V CPU connector, any modern PSU should be fine to use (given it has all of the peripheral connectors one needs for the rest of the system.)
In regards to the Asus P4B533, it's Intel i845 Northbridge, which has a TDP of about 10-15 Watts tops. I didn't look too deeply into the pictures of the motherboard online to try to see if power for the NB is derived from the 5V or 3.3V rail... but in any case, you're looking at either 3 Amps max from the 5V rail or a 4-5 Amps from the 3.3V rail. Both of these should be trivial for any modern PSU. So no need to look for era-appropriate PSUs if you don't want to.
I personally prefer to buy used older PSUs from major OEMs (like HiPro/Chicony, Delta, LiteOn, Newton, AcBel, and Bestec, to name a few), but that's only because I DO actually like to recap them and because I want to help reduce eWaste. Also, I trust my recapped units more than I trust modern units. I consider lack of APFC in the older units another plus, as it usually makes them more reliable in the long term (decades of use.) APFC tends to stress the primary cap(s) of a PSU quite a bit, and the average expected life is typically 10 years (or less with cheaper cap brands.)
