I've got a 230W Antec power supply that works as well with an Athlon mobo as the 400W which I bought because a > 300W was recommended. Judging from the temperature of the air coming out, it is coasting.
Since the ratings put on the side of the power supply are almost totally BS (according to the Tomhardware tests), the buyer must beware. At one time -years ago now- a power supply over 250W was rarity and it was probably a quality one. Not even remotely the case any more.
Judging by what happened during THG tests, neary all PSs must be operating far below maximum capacity. (If they weren't, people constantly would have their PSs catching fire too!) That must be because few of the devices actually pull as much current as the maximum.
I see people worrying about their PS being too small because they have added two fans. Come on, people! A fan usually pulls 3W at max and you've got it hooked to a 300W supply.
According to the info at THG, the CPU is operated from the 12V supply. Amazing. Why, when the CPUs operating voltage is under 2V, and there is a 5V supply with a huge current rating?