The best way to find out what power supply you need is to open your Computer BOX. Now there are only really 2 types of power supplies it could be. Now i would like to know if your PC is old or new? old is the last 10 years new is the last 4 years. If you have only had it for about 4 years you will need a AT-X power supply. Now there is a big difference in old Powersupplies and newer ones. Do this simple test- Open your PC and see how many power cables from the powersupply go to the mainboard, if its a newer board there should be 2 cables going to the main board (one is a huge long one and one is a little 4 pin one that plugs in beside your CPU most of the times), its always black and yellow. Oh Also on the new Powersupplies they have a brake off option on that long plug i was talking about which can take a 24 pin to a 20 pin... GO buy a AT-X powersupply.
DID i write to much?