I am thinking of purchasing the Dell I580-5108NBC, but I want to add 2 1TB SATA drives in a RAID5 config. The system only has a 300W power supply, and already has a 1TB drive and a DVD burner. Is the 300W power supply sufficient to support two additional drives, or should I upgrade it? Or, can I simply unplug the DVD burner?
The most effective way to know for sure is to plug them in and see if you start seeing random failures or some components not being recognized. You could also try a PSU Wattage Calculator.
A 1TB SATA drive doesn't use that much power. For instance this drive uses 5.4 Watts maximum. Call it 7.5 Watts per drive to be on the safe side and to include the power for the RAID controller. You'll need 15 Watts from the power supply.
Power supplies in packaged systems like the system you suggest are generally sized to match the system. This keeps cost down. However, there's likely enough spare capacity as the 15 Watts is 5% of the total capacity.
You can't unplug the DVD burner without opening the case (I assume you want an internal burner). That's not really practical.
I do wonder though if this PC is right for the a RAID 5 configuration.