GPU manufacturers quote stuff like 300W just as a general save their own skins if something goes wrong. Its actually far more usefull to know how many amps the PSU can deliver on the 12v rail.
My PC, which is a 3Ghz Q6700, 6GB Ram, 2 HDD + 1 SSD, an ATI 5850HD, CD writer, floppy! and zip drive still only draws around 120w idle, and 200w or so while gaming, and thats measured at the wall, so really only around 180w DC from the 12v rail (Thats 15amps@12v) probably a bit less, as ram and a few other bits and pieces still use the 3.3v and 5v rails.