I'm a great fan of the call of duty series (mainly modern warfare 1 + 2) but they don't run on linux (MW 1 should but I couldn't get it working).
There for I still use windows.
For the rest, Linux (under dualboot).
Modern warfare 1 MUST BE patched to atleast 1.6 (1.7 is recommended). And will lag if you put everything on the highest do to some shader issues. Otherwise it works fine.
Modern Warfare 2 works if you only play multiplayer as the video thing in singelplayer makes it crash (can be fixed by deleting the videos?)