Increasing the RAM will increase the # of frames that can be rendered. The green bar represents the frames that can be cached in RAM for previewing. You can set the work area to just that many frames so it will restrict the RAM preview only to the # of frames it can do.
Other choices would be to reduce the frame resolution and to change the RAM preview to render every other frame.
Jim Kanter,
Digital Film Institute
http://www.dfilminst.com