As you probably already know, the P5Q SE motherboard supports dual channel memory. For dual channel memory to work correctly you need a matched pair.
You could probably get the 1 GB and 2 GB sticks to work in the P5Q SE motherboard, but this would most likely result in an unstable computer. For most users it is not worth the risk of loosing important work when your computer crashes.
Of course, you could use just one stick of RAM (for instance, the 2 GB stick) and your system will function fine and should be stable.