Tepid and BigDav are spot-on
You only need however much ram is necessary to "hold" whatever applications and documents you have working. That may be 1 GB or it may be 12 GB. (Well, 1 GB may be skimping even for someone like me who doesn't "tab"and doesn't "multitask") The amount of RAM that one needs depends on whether they have multiple applications open and have many documents open or "working" at any given time. If your RAM capacity is not exceeded such that it is frequently swapping files back and forth between RAM and swap files it has had to place on the harddrive, you would not benefit from additional RAM. If one wishes to make a significant increase in computer speed in this situation, their money would be better spent on a faster processor. If, however, one tends to have a lot of applications open and has numerous large files open at the same time they are streaming music throughout the house, they would do well to have a very large bank of RAM.
The situation is kinda like asking how big a truck one needs to haul "something" - - Just depends on how big and how heavy the load is. Likewise one needs as much RAM as is necessary to "carry" whatever "load" they would ask of it. Always remember one point about RAM: more RAM itself doesn't actually make the computer process information any faster, it just makes the raw information more quickly available to the processor.
To answer the specific original question posed here: If you have 8 GB of "stuff" working so RAM is having to swap between RAM and harddrive swap files, YES, 8 GB would speed up your computer. If you have 4 GB RAM installed and you never actually USE more than 3 GB, NO, more RAM will not speed up your computer because, in that situation, the processor really would be the limiting factor. (If a pick-up truck will satisfactorily haul the load you need hauled, there would be no benefit to hauling the same load in a tractor-trailer.)
