I'm not an AE pro but I'll try to give you some advice.
Make sure that in your Composition tab the resolution is set to Automatic so it isn't trying to render full-resolution frames when it doesn't need to.
In the Preview tab (if you haven't already done these things), try setting the resolution to automatic, lowering the frame rate, and maybe setting it to skip a frame or two. This will help for RAM Previews, but not for general scrubbing.
I have a 15" mid-2009 MBP with 4GB RAM and a much slower processor, and I do okay with it. It's always good to be conscious of the complexity of your comp and try to achieve the results you want in the simplest ways possible. You might also consider upgrading your RAM to 8GB. I'm planning on doing that soon and I know it will speed things up a lot.