Here is recommendation list (in this order):
Make sure you have 4GB of RAM. Two 2GB DIMMs are favorable to ensure you can use the tightest timings.
Make sure you pick a branded product of at least DDR3-1333 speed and timings of CL8 or faster.
Go for a faster product if you find DDR3-1600 memory that provides the same timings as your preferred DDR3-1333 RAM kit at only a little price premium. Don’t do it if you could get a faster processor for less or the same extra money.