Adding RAM doesn't speed up a machine, it removes some barriers to speed.
It's like saying taking your foot off the brake speeds up a car. No, it removes the slowdown.
You'll see the benefits of more RAM in situations where you were formerly exceeding the limit of physical RAM -- running multiple programs at once, running programs that consume a lot of RAM, or large data sets. It won't do a thing for the hard drive read/write speed, computing speed of your CPU or your video card, so things that are dependent on CPU, video or disk access you won't see much of a change on.
IF you were formerly getting a lot of pageouts because the OS didn't have enough RAM, there should be a general increase in snappiness in switching tasks, due to not having to go out to hard drive as often.