The 9800 pro is by far the best card there.
Also consider fx5900xt or fx5900 ultra.
For a 9800 pro and the two fx cards i have told you about you will need a 300W power supply (although you might get away with 250W). So check what yours is. PSU upgrade is not difficult and they are cheap to buy (even a 550W is only 20 quid).
As your mobo has a 4X agp slot...all these cards should work.
Like I said...the only issue is that you might have a low wattage PSU.