I had a Dell 300 watt PS that could handle 8 hard drives on my celeron 1.8ghz. (as well as optical drive, dual ethernet cards, integrated video, several case fans and CPU fan, and 2 sticks of ram.) Hard drives may not use any power when Idle, but when booting up, and all 8 of them try to start spinning up at the same time, It puts a heavy load an the power supply.
Unfortunately, it couldn't hack it when I installed the 9th hard drive, so I upgraded to a 430 watt. 🙁