There is a software package....
hyperionics...that will capture your game play. No..not full screen because of hardware, but you can shrink the view and capture the video and sound. I used this to capture a 12min RTCW demo. I was able to capture it at 320x2xx@15fps, low sound quality and standard, I think, compression. It ended up to be a 155mb file with okay qualilty.
Play around and you might get something better...good luck!
-JC