There are a number of reviews out there thatcompare the two cards, do a search and read a couple reviews, you'll probably find your answer. If you have any detailed questions come back and ask, people are much nicer if you spend a little time of your own doing some research.
Depending on your overall system specifications, the GT is significantly more powerful than the 9800. Post your system specs and we will have a better idea regarding your question
In otherwords you may not have a system that can take full advantage of the GT.