Hi Big_Boss
A 300W PSU should be ok ... best thing is to try it and monitor the voltages. If they are within tollerance, then all is well ... if the machine is a little tempromental, and the voltages are low, then you will need a bigger PSU
It all depends on what current the individual items draw, two 60gig hard drives from different manufacturers could draw entirely different currents.
I would normally go for 350W as a minimum for a current setup with all those peripherals, but as you have a PSU, give that a try
Hope this helps