If you look near the top of the window, you'll see a tab called Guides. In there are a variety of articles that answer the questions that are asked here repeatedly. Such as:
by using 2x2gb ram only 3,3gb will be recognized. but will it still use the dual channel?? or is it the same as in using 1x1gb & 1x2gb ram? what is more advisable?
You'll still have dual channel if you have 2x2GB sticks.
One interesting thing I found with Leopard is that in Activity Monitor, the graph of the system Ram shows my full 4GB of Ram, whereas in Tiger is only showed 3GB. It still can't access the full 4GB though