you will need a power supply adapter to use a regular ATX PS on a dell desktop. here is some info. if you plug in the PS without the adapter it will short out the moterboard and possibly damage the CPU.
6 Pin Dell P10 AUX Connector Adapter
if you are going to buy a power supply i would get at least 500 watts to use with video cards. the video card you picked will work with 400 watts
but if you decide later to buy another computer you can remove the PS and put the old one back in. video cards in the $100 plus range require more power.