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It can unofficially take up to 6GB (4GB+2GB) of RAM. You must use DDR2 since DDR3 is incompatible.

The HD bay is 3.5" and you can use any 3.5" SATA HD. However, replacing the HD isn't very easy.
 
RAM for that model is DDR2 PC5300, and maxes at 6GB. I believe some folks have had luck getting it up to eight, but don't quote me on that.

It takes a 3.5" HD, but Apple didn't build these to be user servicable. Have a look at guides on ifixit.com or similar sites to see if that's a task you want to take on yourself.

For future reference, websites like everymac.com can be a great resource to look up more information on your computer.

Edit: oh crap, Hellhammer beat me to it. I'll leave it here anyway.
 
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