I have a 15" Powerbook with 256mb of ram and I'm looking to upgrade to maybe two 512 sticks. I don't know if two is necassary, but I am a graphic design student and I run big programs like Photoshop & Illustrator and I need as much ram as I can get I suppose. How do I install the memory? Do I need a certain type of screw driver? Can I have one 256 stick and one 512 stick in the machine?