Re: graphical corruptions in kde4/qt-apps
I would add that when displaying video whilst using a compositor disables xv vsync... It is slightly better if you choose a ogl renderer for video (but does not remove the problem and uses more cpu power too). I would also point out that with mplayer on nvidia cards ogl produces a much sharper and less blocky image than using xv.
I dunno if op was talking generally or just video (xvideo settings shouldn't change general corruption issues).
p.s. Kwin automatically sets it's framerate at vsync, which explains why it insists on sticking to 50Hz when I have dynamic twinview enabled and the more proper 62 Hz when it is disabled (makes me question what the vsync box does in kwin settings).