no not anymore that was old ATX1+ specs the new ATX2+ specs dictate much less power from the 5 and 3.3 and more use of the 12 V, if you look at a modern motherboard (made acording to ATX2 standards) it has its own switching regulator for the cpu, the video card once run of the psu's 5 and then 3.3 volt rail now also takes its power from the 12 rail via another regulator on the video card, I think this is partly due to the escalation of power consumption especially by once trival things like video cards that now rival the cpu in consumption, running off the 12 volts and converting it onboard means the carrying of 1/3 the amperage as the conversion is done right next to the component needing it. memory now runs at 1.8 volts so another onboard regulator is used and probably for the motherboards chipset too, nowadays there seems to be little that is not run from the 12 volt rail of the psu I would not be surprised to see the 3.3 and 5 volts disappearing totally in the near future. if you have a new pc that requires a 300 W psu an old one will not work you need a new one and vice versa a modern power supply cannot handle older pcs because there is no longer the required amperage on the 3.3 and 5 volt rails