play2loose - u should go over to your friends house, open his case just to look at the card, and verify that its a 9800pro.
then look at his display settings and see if hes running something different then u r... example- is he running it in 16bit res instead of 32? and with Vsync off?
go threw all his settings in windows advanced display, and inside Doom3's settings, and right everything down.
id be curiouse to know what the diff is that makes his 9800 run it better then your system.