finally switched back to using the neuveau drivers which work just
fine
for my needs.
And finally, me too : nouveau was always entirely unusable and very
unstable on this hardware (the "legendary" overheating 8600M GT, see
nvidiadefect.com && wikipedia), up to and including 3.4.7 kernel at least.
With 3.6.6-1 (or earlier), all these nouveau bugs affecting this card
seem to be fixed now, video and opengl look great, xrandr works, system
is even significantly cooler *and* more reactive than with nvidia
driver, which was already "just fine for my needs"... 5-10ยบ and 20%, how
can ? Day 2, counting ;-)