Did you buy this computer or build it yourself?
If you bought it from a company it may have a smaller or odd size power supply.
And before you pick an Nvidia or AMD card. See what games your going to play. Then see what card is better suited for those games. Plus go with the newer generation nvidia 6xx cards or AMD 7xxx cards since they are a lot more power efficient. Also Newegg.com has a power supply calculator. So you can see the estimated power supply use.