I had trackir and sold it (read: gave it away). Reason? No decent headset for under a few $K. 
If anyone buys that new headset please give us a review. If I remember I tried some Sony's but the 
640X res was bad. 
IMHO if you can put together a great headset with a trackir then you have IMMERSION! 
Ren
Aces High Training Corps
ps I should get another trackir just to answer noobs questions. I tend to forget the different setups from time to time.