The RADEON 9800 Pro will make a HUGE difference.
You currently have a GF4 MX...The MX series of cards has horrible performance. (TI series of cards are the better one)
If you up to spending the money for the 9800 Pro, definitely do it, since it's one of the best performing cards on the market. (The XT series is only slightly better than the 9800 Pro, and is not worth the $500)