I would suggest the GeForce 8 series or the GeForce 7900. They should be great for FS2004.
Now for FSX. You don't just need a card for it. You need to know your system pretty well to make FSX playable on it. FSX is one of the most demanding games out there at the moment. Even for current high end stuff you wouldn't get FPS and graphics like FS9. It takes tweaking and a lot of time messing around with it. It isn't as easy as buy RAM and a good card to play FSX.
This is an excellent guide to an FSX machine from our former member Nick N.
http://www.simviation.com/cgi-bin/yabb2 ... 1186945815
FSX needs a lot of work to get it running smoothly. If Nick N were here he would really help. He i think he had a machine that could run FSX pretty well.
Hope this helps.