The x800xl is a damn fine card and will give you a nice boost over your old raddy. I really cant see why it should give any probs going in a Dell box as long as there's nothing encroaching into the agp slot area, even then with a little jiggerypokery (and maybe a dremmel
) you should be ok.
<edit> An agp version might take a bit of hunting down as they're predominantly pci-e, afaik.