P.S. The only thing that went wrong is that the button to open the macbook is not working perfectly, i have to press it harder than usual, i guess i have to disarm it again and check that everything is in place, other than that it is working flawlessly ;)
Thank you all very much, this guide is perfect for everyone, no matter your level of expertise, i never opened my mac before and i succesfully replaced my old 120 gb HD with a new 320 gb HD 7.200 RPM :) And worked flawlessly!!
BIG UPS to ifixit, this guide is perfect for everyone!