More RAM does not really affect the "speed" of your computer until the point where you have used it all up and the computer has to start moving things from RAM back to permanent storage.
If your normal usage uses 3GB of RAM, then upgrading to 8GB will have no effect.
The main reason to go ahead and get it is because it can be hard to predict what your needs will be in 2-3 years especially as new OS updates and other apps come out that may or may not use more RAM.