You are in desperate need of answering -- in this very forum, where you stated your problem -- the questions you will find here:
Reduce the number of cores to use.
My situation began with the "RAM Preview needs 2 or more frames to playback" after upgrading to CC. Previous version of CS6 never had this issue on the same machine. Having the same issue as vincent, with max allowed memory registering at 0.4GB in the upgraded CC. Just upgraded to the latest version of AE CC through cue. I have 32GB of RAM in my machine and I'm not currently running any other applications.
MacPro 2x2.4Ghz Quad-Core
32GB 1066 Mhz DDR3
Running OSX 10.9.1
Version: AE CC 126.96.36.199
Just turn off MP rendering and see if that helps. MP rendering only works well in a few situations.