I assume you use some kind of glasses/goggles that let you view the game though them?If not, that's kinda silly, because if you were to say look up, you couldn't see the moniter anymore.
There's also a few free software alternatives but they require webcams.http://www.free-track.net/english/ (Requires you to build a headset)http://sourceforge.net/projects/facetracknoir/ (Doesn't require a headset, But I couldn't test it because it's pretty CPU intensive)
I'd get Track IR because a lot of the games I would play support it, allowing better movement.