Well, About 3 weeks ago, I got a mac mini refurb to hold me over till I get the new macbook pro. I ordered 2gb ram with the intention of adding it right away. Man was it a pain in the butt. I have built several PC's, but this was a b***h. Not that I care, but I cracked some of the plastic on the bottom as I could not get the thing open.
My advice, is to use two putty knives, one on each side. Both should be metal, not plastic. You will have to bear some scratches and cracking noises, but if you are (more) careful, nothing should break.
Once it was open, it wasn't that difficult to get the cd/hdd out of the way, just use a magnetic screwdriver.
The only issue i had was that the airport antenna came off of the airport card, which took me a bit of time to reconnect, but once fixed, it was done and simple.
Have patience and be careful, you should be fine. If you have not experience at all with computers, you might want to bring it to a local computer place and eat the $45 it will cost to get it installed.