Many games nowadays come with a Linux version as well (UT2003 and UT2004) and many games have special mods that allow them (windows-based) to be run in Linux (i.e. Quake). You may want to check that out (Sorry, not sure about Call of Duty). As a third option, Wine comes to the rescue!
Also, like the above poster mentioned, be sure to download the latest Linux driver for your ATI card! I have an ATI 9100 and just finished installing the driver: WOW! OpenGL rocks now! (I also am using Mandrake 10.0)
Hope this helps,