One thing that may or may not help some is simply defragmenting the partition that you have Doom 3 installed on. The install for the game dumps quite a bit of non-continious data on your hard drive. There's also cache settings within the game's config file that you can screw around with that may help, though personally I didn't get any real results out of that.
Both would probably help. What quality settings do you have it on? High works best on 256mb cards. It seems that with enough ram though the problems are minimized (although I don't really understand why that is...unless theres so much data that it has to swap with the hard drive some).