I recently bought additional RAM for my Macbook Pro (from 4GB to 8GB) and had seen some improvement in speed (I have Premiere CS5). Of course you can’t expect to miraculously improve the performance of your laptop, but at least you will be able to run more apps at the same time. Also in Premiere Pro go to memory preferences and allocate more RAM to Premiere and less for other apps. This will help Premiere work faster.
