Everything you view on the AE timeline must be loaded into the RAM buffer before playback. So no, you can't bypass the RAM preview functionality - it's integral to the way AE works. RAM previews should remain intact (green line) so long as nothing else in your composition that effects that part of the timeline has changed, or until AE requires the RAM for other purposes. Sometimes, surprisingly simple associations can be overlooked: for example, if you have a Wiggle expression applied to one layer, and you add a new layer to the comp, the WIggled layer will need to re-render, because the Wiggle expression uses the index of layers to randomize it's output.
Enabling disk caching may help to speed up RAM preview builds, depending on the content of your timeline. Also note that you can double-click a footage item in the Project window and play it in real time as a standalone file, disk speeds permitting.
That doesnt really address the issue of how to get around rerendering everything when i do a simple thing such as move a clip along the timeline.
As I said, RAM previews are integral to the way AE works. After Effects plays composited material from RAM, not source material from hard drives. If you require instant viewing response times, you need to use an editing application, rather than a compositing program. The typical workflow involves editing source files in an NLE, then compositing/mo-gfx with the resulting edit.