I have a Mid-2010 17" MacBook Pro with 4 GB of 1066 MHz DDR3 SDRAM (PC3-8500) installed in pairs (two 2 GB modules). I want to upgrade to 8GB. Do I need to buy 2 x 4GB 1066Mhz DDR3 SDRAM? Or do I need 2 *further* 2GB modules? Can I get faster RAM than 1066Mhz DDR3?