Thank you for your comments.
Yes my AA-settingss are 3 and in Ray trace objects 3, too.
I don´t know what this means. But today i know.

Detail: 0.6
GI: 2/4
At work i have an intel dualcore E8400 @ 3,6 Ghz and at home i have an AMD PhenomII X4 920 4x2.8GHz @ 3.03 Ghz
But the amd was only about 1minute faster than my intel.
@ njeneb: Yes, I saw it only after the rendering that I flew through the trees. For that a GPU-preview would be nice. (DirectX or OpenGL)