You better have good airflow in your case if you get that cooler. These fanless coolers assume that you have a few fans in your case which can actually take the heat somewhere else. Otherwise you won't accomplish anything, and your video card will bake in it's own heat.
your 9800gt is 100W at most. i have an arctic cooling accelero s1 rev2 on my (overclocked) 4850 (>110W)with 2 12cm fans on it (with zipties ) and i get 60 degrees in furmark during the greek summer
also, do not put a stock cooler for a 9500GT (50W card) in a 9800GT (100W card).