I've downloaded the lost coast as well, and it runs fine on my 9800Pro. a little slow, but it does run.
Your 9800Pro card is already DirectX9 compatible, is the game not allowing you to run, or is it just a warning. I get a warning, which I then just ok, and it lets me run the lost coast extra level.
If it won't let you run, then on the ATI site, the catalyst drivers are compatible with all radeon cards.
So you just need to download the drivers, and install them. Some people say un-install the old one's first but I never have, and it's never been a problem.
However, if you are unsure, then un-install the old drivers, through add/remove in the control panel. Then restart, cancel the found new hardware, and then install the new one's.
P.S. it's very beautiful the new level, but it does make me want to upgrade my graphics card to something a bit better :)