I see macsales.com has a special on 2 GB ram for my AL iMac. http://eshop.macsales.com/item-specials/Other%20World%20Computing/5300DDR2S2GB/ Could I buy two of these and install and have 4 GB????
OWC (MacSales) is a highly respected Mac upgrade seller. Also check out Data Memory Systems When buying RAM, choose a reputable dealer who tests and guarantees compatibility with your model Mac, offers a lifetime warranty, and a no charge (no restocking, no shipping) return if the RAM doesn't work.
Actually, it is in the MR Guide - Buying RAM But it's more fun to type it out... So many newbies, so little time MacNewbie03 - welcome to MR. You'll find many answers to common questions under the Guides tab at the top of this window. The second half of your question is addressed in Understanding Intel Mac RAM
If you're going to get 4GB, just buy set of two: http://eshop.macsales.com/item/Other World Computing/53IM2DDR4GBK/ Savesyou whopping $0.50