It is probably not the best idea to run high demand games on the same computer you do media on. There are a lot of settings that make games look better(particularly in the graphic domain) that should be set to low or even turned off when running media programs (eg ProTools, Cubase Sonar. . . etc). Lots of the tweaks to improve the media performance can be found here:
http://www.musicxp.net/tuning_tips.php
I suppose you could set up multiple users on the computer and have one tweaked for gaming and the other for media. However some of the tweaks are global (not specific to one user) which means either games or media programs will not be running totally optimized unless you go through and change all the settings when you do one of the other. Of course, if you get a blazing fast computer (core duo, 2GB Ram, yadda yadda) you might be able to run the media without much trouble without the tweaks.