When installing new ram you've gotta think of a few things.
First, you can install any different sizes together (as long as the total doesn't go above the maximum ram amout of your motherboard)
Other than that, make sure the RAM is the same form factor. They have to be the same as your mobo. SIMM/DIMM/RIMM
The second is if at all possible, use the same brand of memory for all your ram. If not, make sure that the pin connectors are the same color (IE gold or silver).