MacBook Pro - How to upgrade Memory-Which type?

I purchased a C2D MBP last fall and because of the extra $500 or so for 3GB chose to go with 2GB. I'm sure memory has gone down in price some and most of all it's cheaper to buy elsewhere than Apple.
My questions:
Since I have 2GB of memory would each slot be filled up requiring me to purchase one 2GB stick to use with one of the 1GB sticks already installed or would it be just one 2GB stick allowing me to get a 1GB stick to add to it?
Is this something that is easily done by the user?
Most of all, what is the best memory out there? I want a reasonable price but don't want to buy anything that will cause stability issues.