Oh man...its going to be hard to upgrade. The thing is, dell uses custom Dell-made powersupplies, motherboards, and fans. First off, make sure your motherboard has the appropriate slot for the video card (a newer video card will most likely be AGP 8x/16x or PCI-E). Next, make sure you have an appropriate powersupply open in your computer, since you really can't upgrade the powersupply in a dell. The only problem left is that since all dells ship with one fan, and are built around it, if your video card cant keep cool on its own, you really can't intsall another fan.
I had a Dell a few months ago, tried to upgrade it, ended up buying a whole new comp because i was so pissed. Got one custom built, and now I can customize all I want