I ran a 6800GT AGP in a Dell like that. Even a ATI 9800 Pro would be an upgrade to a FX 5200. The Dell had a 9800 Pro and the 6800 GT was a nice jump.
My old Dell with a 250W needed a PSU upgrade for the 9800 Pro. It was too close. I'd assume the same for the 6800/7800 recommendations that followed. Funny thing is, I still had that upgraded PSU (Antec 300W) sitting around until a week ago, or I'd let you have it for the cost of shipping.