Let me also suggest a few other basics for laptop owners that will help with overall performance and, ultimately, shatting your pants over this awesome game:
1. Clean out that dust! Be sure that your laptop is free of dust and other airborn nasties by uisng a can of compressed air (available at any computer store) on the intakes that are underneath and on the sides and top of the laptop. Build up of dust can happen far into the laptop's ducts and will cause air to flow less freely, ulitmately causing excessive heat which will throttle back your video card's performance threshold.
2. Cool Pads are kewl! Don't be a Nancy, shell out the $29 and pickup a laptop cooler. This nifty little item will keep fresh, cool air flowing into your laptop's intakes therefore keeping the video card (GPU) cooler and frame rates kickin'!
3. Defrag your drive. These levels are huge and take a fair amount of time to load, even on the 7200 RPM Hitachi drive. Defragging your hard drive will help keep load times down to a minimum.
4. I highly recommend headphones for this game. Most of the spookiness in Doom3 is from visual and audible subtleties that are most efficiently recreated with a nice set of cans. With the quality of headphones so good these days, there really is no reason to have a set anyway.
5. Turn out the lights!! Man, I can't tell you what a difference having the lights our makes for the highest possible "Scare Factor" in Doom3.
6. Overclocking for the most frames per second. There are many discussions on overclocking video cards on these forums so all I'll say here is pick a program you're comfortable with and get the most out of your card for this resource hungry game. The laptop cooler will make overclocking much more successful as it will keep cool air flowing freely.
Remember that you're only going to get to play this game ONCE for the FIRST TIME. Make that experience worth every penny of your hard earned cash. Turn out the lights, overclock that video card, put on the headphones and hold on! Oh, and have an extra pair of shorts out for the first time you see a Pink Dog!!