Fraps is the de facto standard in "recording" screen shots, though the freeware version has some serious limitations (the maximum duration of 30 seconds is a quite severe one).
Using an "analog loop" (i.e. cloning and connecting from the TV-out to a TV capture card and start recording) is probably also an option, and so would it be to start a VNC server and have a VNC client record the screen buffer, but both sounds like over-kill situations. The later is probably not even capable of giving you reasonable frame-rates.
It's the frame-rate problem that's really the show-stopper here (excuse the pun!). If I recall correctly, one of the more famous freeware AVI/MPEG editing/filter/converter programs (VirtualDub? FlaskMPEG?) should have some screen capturing capabilities, but it's not nearly as smooth as the good old Fraps.
Edit: Please note that all the comments are from a Windows point-of-view. It's highly likely that *nix has some neat X11/kernel module available capable of doing what you wish to do and that someone else could easily point you to.
