I have the 2.16 GHz Intel Core 2 Duo, it is 2 1/2 years old with 2 GB 667 MHz DDR2 SDRAM and 160 GB toshiba 5400 RPM HDD (apple just replaced cause my 120 GB HDD just failed) Can I upgrade my RAM to 4 GB? Can I get DDR3? Has anyone done this? Will it make a difference? Any site where I can get the RAM relatively cheap? Thanks!