Since you'll have to open the computer anyway to install the video card, you might as well have a look at the power supply and slots now. There'll be a wattage maximum listed somewhere on the unit... "MAX LOAD 300W" or something along those lines. While you're in there, check for slots looking like the ones I posted in the picture... white ones about 3 inches long are PCI, 1 inch long colored slots are PCI-E, and the longer (4 inch or so) colored slots are PCI-E x16, which is what you need for a modern GPU.