How much does it cost?
I recommend a 6600GT video card. It should costly almost the same, and you get a lot more performance.
The two bad things are these:
a) You must have a good, quality 300-350 power supply, from a good, well-known manufacturer, with at least 20 A on the 12V rail, in order to have a stable computer.
b) The performance of a 9800 PRO or a 6600GT will be limited by your current CPU (is it a Celeron D or a Celeron?).
Good luck!