Check where the main input cable goes, and there should be a big fan beside it. Take off the Computer side cover and there should be a box 9 x 6 x 4". On the side there should be a description, like 'ATX 12 volt' 'with 'PFC' etc. Then there should be a data sheet in small writing with wattage, etc. beneath the description. Depending on what card you bought, think about a minimum of 400 watts.