Sorry for mistakes, I'm German! 
My RAM preview is not limited to my work area and I couldn't find any way to change this behavior. Is there a preference to check or any other solution?
I'm using (I don't know, which information is needed, so I'll list everything I have in mind):
Windows 7 64bit,
After Effects CS5 10.0.1.19,
Intel Core 2 Duo 2.4 GHz,
GeForce 9300M GS
It would be great to get an answer, so I thank you in advance!