Why does AE CS4 consume all my memory when running a preview?

Question

I have Windows Vista x64 with 4Gb of RAM but everytime i want to make a RAM PREVIEW the program does not release the memory in use. For example: I start with AE CS4 using 2gb of ram, then i make a preview and it reaches to 3gb of ram. If I continue making previews it comes a time when all hangs up becouse i have no more memory left!
I remember from previous versions of After Effects that the "green bar" over the timeline, when you were lack off memory, it started to move at some point and the older rendered frames start to disapear in order to render the new ones, making the lenght of the green bar as long as possible but without taking out ALL your memory.
I've found in Premiere CS4 an option that says: Optimize Rendering for Memory/Performance. This help me out with the same issue in Premiere, but I can't find anything like this en After Effects.
Any suggestions?

Answer 1

Take a look at the memory and cache settings. Try increasing the amount of memory to leave for other applications. Also, MP4, due to it being long-GOP, does require a higher memory footprint to decode than some of the other formats. AE is designed to use as much ram as you tell it when doing a RAM preview. Depending what else you have running on your system, our defaults may be too much.
For instance Internet Explorer is taking 100 MB just to show this page. If you launch task manager, you'll need to add the Working Set column to get an idea of how much RAM something needs.
--chris