A very good option to look at is dvGarage's dvMatte Pro for FCP.
It's relatively easy to use and claims to have fast renders. I personally haven't used it. We don't do green screen here yet... but if we did I would seriously consider this program. Download the demo and see for yourself.
http://www.dvgarage.com/prod/prod.php?prod=dvmattep3&sub=features
_______________________________
Wayne Carey
Schazam Productions
http://web.mac.com/schazamproductions
schazamproductions@mac.com