Hello. I'm gonna buy the iMac 21.5" inch one with the i5 2.4Ghz, And I've heard upgrading the RAM to 8GB Is too expensive on the Apple stores, So I've been wondering what kind of RAM kinds do you suggest me to buy if so? Please offer me more than kind of RAM if there's. Also, How hard is it replacing RAM for an iMac? Can I do it by myself with a tutorial or I must let Apple do it? Thank you very much!