I think what was actually being suggested was upgrading the internal power supply to 400 watts, like this:
http://www.cputopia.com/psu-replace-400-p-dell-755-mini-tower.html
It looks like the mini-tower has the 305 watt power supply, so not sure if this is needed.
http://www.cputopia.com/psu-replace-400-p-dell-755-mini-tower.html
It looks like the mini-tower has the 305 watt power supply, so not sure if this is needed.