here's a question for you guys:
i too have a 8400M GS (Dell XPS M1330), using it on Xubuntu with Compiz-Fusion and Twinview. i use a Dell 2005FPW (1920x1200) as second screen on the HDMI port.
overall, i must say, performance is pretty decent. i have the big desktop, Compiz running on 60hz with --loose-binding and vsync enabled, Powermizer at max. animations are pretty smooth and constant. the settings i currently use are the best combination i could find, and i tried a lot of them (in both nvidia and compiz).
now here's the deal: on the external screen there is 1 (one) tear/scanline, which seems to run down the screen very slowly. when it's on the bottom of the screen i can wobble around a terminal window on the top side without any tearing or corruption, but as soon as i move over the line the window gets 'sliced' and both halves are dispositioned slightly. this behaviour is visible in every animation, like wobble, expose or cube rotate.
i don't know if this is the same tearing issue being described in the rest of the posts in this thread, but what i'd really like to know is what the culprit would be in this scenario:
1) 8400M GS gpu speed
2) 8400M GS memory (128mb)
2) driver optimization
3) compiz optimization
4) monitor sync(?)
(heightening compiz framerate above 60 doesn't improve performance whatsoever)