First of all thanks for the fast help!
Now I played a little bit with the ingame settings and after I put the "space detail" off, the game became more smooth. So I installed Fraps and started the game and turned the space detail setting on high again. In normal space i run with constant 6o fps, when i reach a dust area with clouds my fps drop to 30 and the deeper I fly into that area, my fps reach 15. Now I once again turned off the space detail setting and flew into the nebula. My fps dropped to 30 and stayed constantly on 30. Looks like there is no better solution and my graphics card won't handle it better. What a shame...
But as long as Freelancer is playable, I'm happy So thank you!