Memory Type: DDR3 PC3-8500, DDR3 (non-ECC)
Maximum Memory: 8GB
Although the memory can be installed one module at a time, the best performance comes from using matched pairs of modules.
Memory or DRAM is the "working" memory of the computer. It's used to store data for programs (sequences of instructions) on a temporary basis.
Install Guide for your:
MacBook Pro 2.66GHz Intel Core 2 Duo (15-inch DDR3) Mid-2009
video - installing memory