I can't remember off hand the exact numbers, but I used to know the exact power needed for each component, something like:
PCI cards = 5W each
ISA cards = 3W each
AGP cards = 10W+
CDROM(optical) = 15-20W/doubled on startup(spinup)
HD(mechanical/magnetic) = 20-25W/doubled on startup(spinup)
various fans are around 3-8W each usually
CPU = anyone know?? I'd guess around 10-20W for Intel, 40W+ for AMD
Add all your stuff up, power supplies spike on startup for a few seconds and give their max rated power only for that short time, sometimes even a little more.
Normally, a 300W runs at about 200W-230W once the machine is up and running.