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Why are you trying to max out the RAM? What kind of applications are you using on the machine? Chances are you might be looking into a processor upgrade and therefore a new machine.

My advice (and I'm sure I'm in good company here) would be to spring for the 13" MBP if you can afford it. Great little machine that's still very compact and portable yet has enough juice to handle some more intense applications like Photoshop and even SD/basic HD video editing in FCP.
 
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.
 
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