A standard Antec PP303 300w should be fine, unless you plan on loading it up with tons of devices, like a huge RAID array. Anything higher is usually overkill and unnecessary, so long as you have a quality unit, so avoid brands like Deer and such. Only get something with a higher wattage rating if it will make you feel more comfortable, or you like the features from a more expensive PS unit (extras like a switch which let you manually control the fan speed, etc.).