Did After Effects CS5.5 ever work on this system? Is this problem only happening with a specific composition, or with all compositions? Is After Effects CS5 working fine on the same system? What are your Memory & Multiprocessing settings? Are you using OpenGL for rendering? Does a standard preview (when you press spacebar) work OK?
By default, in CS5.5 the disk cache is turned on and points to your user directory. If that is on an SSD or you have some other issue with your harddrive, that might explain it. Look into that and yes, as Todd said, MP settings and so on might be relevant, too.
MP setting were as I have in CS5 (which were fine).
The problem was from the beginning. Simple 3D animation, the moment I use 32bpc and switched motion blur, RAM preview takes ages to render it. (in CS5 usually 3-4 sec). CPU usage was 99.99%. Looks like AE renderes 2-3 frames and freezed whole system, but when (after 1-2 minutes of waiting) I pressed SPACEBAR usually I can noticed that the RAM rendering was done in 30% (so in the background the rendering is working but "very slow" and uses lots of CPU).