Dell's desktop often use a propreitary PSU.
I looked it up this is the shape of your PSU: http://www.amazon.com/Genuine-Dell-Optiplex-Dimension-Systems/dp/B0029SK82G
Normal PSU's are recutangluar not shaped like that. You would have to find a lower end video card to put in there. I doubt that PSU has any 6pin PCI-Express connectors (not to mention the low wattage) so you would be stuck with an ATI Radeon 5670 or a GT 430. But those video cards would probably put to much of a load on the PSU.