Well that means you are using a Socket 478 motherboard. The problem with Dell cases is that they usually have a propriatary power switch and other components. It's a real pain to replace a motherboard in a Dell, especially if it's a small form factor one.
From the looks of it the best card out there that's both low-profile and is AGP is the 6200, which isn't a very good card. It will output to an HDTV with Component cables though, so it'll be accepatble for 720p HD output to a HDTV, but you won't really get beyond that.http://www.newegg.com/Product/Product.aspx?Item=N82E16814127203