I have that machine and it's rock-solid with 2GB + 1GB.
I suspect the instability might not be to do with having 2GB + 2GB per se, but rather that perhaps your new RAM isn't good quality or is faulty, or there's some other hardware fault.
What brand of RAM did you buy?
I've heard (but don't know if it's true) that quality (more expensive) RAM works better, e.g. Crucial brand in the UK, no idea in the rest of the world.