The screws on the back of the unibody require a Philips head #00 screwdriver.
Do not use the driver to actually install the ram. Use your fingers or a plastic tool to unlatch the side tabs. If you break those tabs Apple will charge you a BIG fee to fix the machine.
You need a Torx #6 driver to replace the lugs on the side of the HD if you want to change the HD.
Changing the HD or Ram chips will not void your warrenty but the warrenty will not cover any replaement parts or damage caused during the installation process.
So..... be careful.