the app 'ffmpeg' is better simpler to record desktop than istanbul & those that rqr apache,servers,clients,vnc etc
but it's ugly cmd line stuff

it rqd a ffmpeg that is built to accept the x video screen as the input device
 see http://ubuntu.wordpress.com/2006/06/08/how-to-create-a-screencast-in-ubuntu/

say yu have an app that you want to record...

you likely have 2 ffmpegs, but one is isolated in a spcl dir ( the one that can capture xvid )
go there, & find the location & size of the already open window you want to record

xwininfo -frame
  this rtns a load of stuff including the x,y posn and the width, height size
you plug those info into this incantation
                                                    x,y      w x h    yourfilename.ext
./ffmpeg -vcodec mpeg4 -b 1000 -r 10 -g 300 -vd x11:0,0 -s 1024x768 ~/test.avi

it looks like this...

cd
cd software/ff<tab>
./ffmpeg -vcodec mpeg4 -b 1000 -r 10 -g 300 -vd x11:0,0 -s 1024x768 ~/test.avi

the recording begins immed capturing the xywh you specified,
go to theat window and do stuff
then go back to the terminal you used to issue cmds, make it active & type 'q'

the file is closed & ready for viewing or furthre processing


i didnt try sound yet