@kdfrawg This from OWC: https://eshop.macsales.com/shop/memory/MacBook/Pro/Core2/ says while Apple claims 4 GB, they have tested it with 6 GB & it's OK. Mine is the MacBookPro4,1 model, 2.4 GHz C2D, originally with a 200 GB HDD - which still works on backup duties in a Firewire case for the 12-in PowerBook.