The model 5,1 (Late 2008, Aluminum Unibody) can use 8gb of 204 pin 1066mhz PC3-8500 RAM.
These are good online stores for Mac compatible RAM
OWC http://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade/DDR3 - They offer Mac tested RAM at very good prices.
Crucial Memory http://www.crucial.com/ - good place to buy RAM from all over the world. They also have an excellent memory selector that allows you to choose memory based on your computer's model
Data Memory Systems http://www.datamemorysystems.com/apple-memory.asp - another good, cheap place to buying RAM if you live in the U.S.
Here are video instructions on replacing the RAM on the Aluminum Unibody.
If you don’t have the tools to open up the MacBook OWC has a set for $5.
Also check out the OWC article