Anyway, your 9800 is a pcie 2.0x16 card so i assume you have a pcix16 slot.
I would stay away from the 550 but go for a 460 or 560, but i think for the last one you'll have to overclock your cpu. ( not sure )
460 is better than the 550. 560 is better than 460 but again more expensive.
If you have a quality psu 450W would be ( just ) enough.