reset the memory usage to default in the edit>preferences>memory and cache screen and turn off multiprocessing - you don't have enough ram to benefit (need ~2gb per processor core).
ae will still use both processors, but in one instance of the programme.
in preferences>previews, try turning off open gl acceleration, it only helps with workstation cards, and even then causes tons of problems.
at the bottom right of your comp viewer, there is a button that looks like a monitor with a lightning symbol in it - click that and set it to fast preview-adaptive resolution.
again at the bottom of the comp viewer, there is a drop down box that lets you choose the preview quality - full/half/quarter/custom. set it to the lowest resolution that allows you to see what you need to see... if you're fine tuning motion, turn it right down.
if you have 3d, lights or depth of field, hit the fast 3d button at the top of the comp timeline (near the enable motion blur toggle), this temporarily disables these functions to speed up rendering for preview purposes.
and obviously, if you have any heavy fx/filters that only need to show in the final render, disable them in your comp to speed things up.
then get the manual out and read up on composition proxies.