Someone on https://bugs.launchpad.net/ubuntu/+s...iz/+bug/151674
found a solution for the video tearing (he was referring to the video on TV but it's the same fix for using a monitor only)
"I had same issue with tearing on TV, but it disappeared after launching the Compiz settings manager (ccsm) on the secondary (TV) screen using alt + f2 and ccsm.
Relevant settings are:
-detect refresh rate off
-refresh rate fixed to 60
So run ccsm on the xscreen where you see the tearing, that is if the tearing happens on your TV and you use the "separate x screen option" in nvidia-settings, run ccsm on that screen and enable "vsync to vblack" ("general options", then go to "display settings" tab)
This fix worked for me on ubuntu 9.10 (karmic koala) - mplayer
No need for you to disable compiz in order to get rid of that annoying video tearing.