Video card

From what I can tell that computer has a 300W Power supply (hard to find out exactly). Probably the best card you can get for it currently is the Radeon HD 5670. Anything more powerful, you'll probably need to get a new power supply. I don't know what FS 2004 performance requirements are, but I would guess that the other thing that will help you is to get more RAM, as the 512mb that is listed as the stock amount for that PC is pretty low for normal usage now. I don't know how much you have, but having at least 2Gb would go a long way to making that PC more powerful.