well.. high performance computers for gaming and other media need more watts for their parts. theres also 650, 660, 680, 700, 750, 850, 875, 1000, etc.. higher than that like 1500 are for networking (http://www.tigerdirect.com/applicati...078767&CatId=0)

however, with older computers, salvaging one of their power supplies might be optimal... an early 90's computer can be 300w or less