Holy moly, that cpu is more than 10 years old. I think you need to upgrade your cpu/mobo before even doing a gpu. I could be wrong though, but man, 10 years is a long time in technology time.
External graphics card? Do you mean out side of the pc case? Or do mean a dedicated card as opposed to on board graphics? If its the latter, you have to see if there is a slot for a card, and tell us if its agp or pcix.