depending on your system configuration:
for PC application (non OC purpose)
1. even the CPU (single) up to P4 2.8GHz and with all heavy loading components such as VGA card, sound card, CD_RW, SCSI HD*3...., the maximum power will be around 150W, but the peak power should up to 250W especially in initial stage
please refer to Intel PSDG
http://www.formfactors.org/developer/sp ... PSDGV1.pdf
2. P4 change from VR8 to VR9, that means +5V will be no more important, even a 550W dual Xeon only need 5V/20A 3.3V+5V<140W, the reason why to keep 5V/30A in PSDG just cover old P3 (VR8 use 5V for CPU), the large 5V will make the P4 system unstable (normally P4 MB need 5V<2A)
3. Check the peripheral connector on your PSU, if you want to draw 300W+ from your PSU you might need more than 8~9 peripheral connectors.
4. The advantage to use a 300W+ PSU: more capability and reliability in derating mode
5. The disadvantage to use a 300W+ PSU: no protect function for OCP, OPP
6. Intel PSDG 300W has 440W+ capability (3.3V/28A, 5V/30A, 12V/15A, 5Vsb/2A) and the protect limit in 300W, that's the advanced power sharing design and meet the real system requirement, just like the pipe and highway design to deal with the dynamic flow
for Server application (or work station)
1. Please refer to SSI PSDG (EPS12V)
http://www.ssiforum.org/docs/EPS12V_Spec_v1_6.pdf
2. All the server use VR9 (12V) for CPU design to deliver high power but keep the similar wire harness
3. The 12V could deliver 240% power than 5V in the same wire
4. The excellent capability of 12V could handle more than 20 SCSI HD for server requirement
5. SSI connect change from 20P+4P to 24P+8P, 3 more 12V wire could deliver 216W safely (each wire could deliver 6A safely but 9A for capability)
then you will know what's the problem of a 500W PSU could provide only 300W capability for ATX wire if you don't have 10+ peripheral connector
The PSU could not provide more power if the system doesn't need it, just like a 911 twin turbo could output 500HP but with a output limit in 100HP.