I use Crucial because the prices are right, and you can run their online scanner. Check it out. RAM Memory Upgrade: Dell, Mac, Apple, HP, Compaq. USB drives, SSD at Crucial.com
If you have dual memory, which you should have, always replace the Ram stcks in pairs of equal size. And keep them in one channel. Say you have two blue slots and two black slots. You buy two 4 GB sticks. Put them both in one color, or one channel. 2x4 GB=8 + 2x2GB=4, totals 12GB. Since I upgraded mine from 6 GB to 12 GB, it flies! LOL I'm thinking of going to the full 16 GB, next.
Have a great Easter Sunday!