Lost_luggage34,
300W is a little under powered for that rig, if Mac were to go to a GeForce4 Ti or GeforceFX Vid Card he would need a bigger power supply.
Here is a good read:
Choosing The Right Power Supply
This is a website I use for power supply requirements:
Power Wattage Calculator
I would say the minimum power supply for that computer is 320W + the additional fans. I always suggest a 400W Brand Name Power Supply with a minimum of 28a on the 3.3v RAIL and a Max Combined Wattage of at least 200W.
Take Care,
Richard