Most Dell Dimensions have integrated graphics...which mean there is no AGP slot on the motherboard. Take down the Service Tag number on the back of your computer, and go to
http://support.dell.com and enter in your service tag #. You should be able to find documentation or something that will tell you if you have an AGP slot.
One good test is to look where you plug your monitor into. If the VGA connector you are plugging it into is on a card, then you're probably OK. If the VGA connector is around the same place where you plug in sound, printer, usb, etc...then you more than likely have integrated video, therefore your Nvidia card will be of no use to you in that machine.
Best of luck