Significant improvement in the speed of execution software in use.
Little improvement in web browsing, especially where video playback is the video card that plays a major role, and unfortunately it is a little dated on portable iBook and PowerBook.
Overall, investing in RAM for these computers today represents little interest. With only 512 MB, it is already working correctly. However it can be useful if you plan to install Mac OS X Leopard (10.5) for example (minimum recommended: 867MHz processor and 1GB of RAM), even if the graphics card sin by his lack of power. By cons, Snow Leopard and Lion, RAM or no background, no need to think about it because it is necessary to have an Intel-based Mac.