On the Preview palette, set the resolution to Half. That should quadruple your previews. Also, according to Adobe, you may have to increase the Maximum allocation per CPU to at least 1GB if working in HD or with large pixel sizes.
“Minimum Allocation Per CPU
Specifies the minimum amount of RAM that will be allocated to each background process, each of which runs on its own CPU (processor core). Setting this value lower can allow more CPUs to be used simultaneously with a limited amount of RAM. However, if you set this value too low for the kinds of frames that you are rendering, then the background processes will fail to render frames at all, and only the foreground process will be used to render frames. For example, you should not set this value to 0.5 GB if you are rendering frames with the pixel dimensions of high-definition television or digital cinema.”