Hi,
You need to factor in the wattage all your components are drawing.
P4/Athlon XP/A64 - 60-90 watts
Motherboard - 20-30 watts
RAM 20 watts per 256MB
PCI card - 5-10 watts
AGP video card - 20 - 50 watts
Hard drive - 5-20 watts
Optical drive - 10 -20 watts
Case fan - 1 5 watts
Tally up what you have running and be sure to leave yourself a generous headroom such as 30% or so for the large power spike when you first power up your PC and everything starts up at once
The quality of the PSU should be taken into account as well. Cheap low quality PSU's won't put out near the power they're rated for when they reach normal operating temperature. Good PSU's are made by the likes of Antec, Enermax, Fortron and PC Power & Cooling.