You could look at the manual/website if you remember the model of the PSU. Otherwise, just open up the case and the wattage should be listed on the side of the PSU. It only takes a few secs.Thanks for the reply. Is there a way to tell how many watts my PSU is on my computer without haveing to open the case?
Remember you will also need to know how many amps are on the 12V rail(s). You can tell how many amps/watts you need by looking at ceewi1's Power Recommendation by Video Card!
You can check http://www.computerforum.com/90117-ceewi1-s-psu-recommendations.html for good quality units. Someone else might have something particular in mind.