thanks for your support.
is has definitely to do with the graphic card settings.
when i switch to per application settings it works fine. (unfortunatly there is no option to save these settings for specific applications)

the graphic card is a ATI Radeon HD 2600 XT and part of a MacPro where i run windows7 on.
the card has also caused some trouble in another 3d app (bonzai3d) and i have yet to find good settings where everything works fine and i have good AA.
maybe it is a macpro related problem. (as their cards might be a little different than the normal pc versions)