You want to leave some CPU's for other apps. Do that by going into the preferences and making sure that you have enough ram allocated for other apps. You usually don't gain that much by throwing everything at AE and leaving just 1.5GB available for other apps. On my most powerful machine I am leaving 4 GB for other apps and assigning 12 to AE.
The use of that memory depends on what plug-ins and effects you are using. It looks like your comp is just an extruded text layer. That's not going to use much ram for processing. Throw in some particle systems, some motion blur, and a few other things and your ram use while rendering will go up providing those plug-ins are written in a way that will allow them to use a lot of memory. I hope that makes sense to you. AE isn't going to tie up any more memory while processing frames than it needs to. It also is not going to load frames into ram for a ram preview until it chokes all other processes.
One thing that AE isn't very good at (but it's better than it used to be) is releasing memory. There's a free app called Free Memory that I usually have running. Sometimes it will unclog the system when the available ram for ram previews gets hung up.