I installed 8GB in my late 2008 Unibody Macbook. Make sure you have the appropriate firmware upgrade:
http://blog.macsales.com/9102-secret-firmware-lets-late-08-macbooks-use-8gb
I first went from 2GB to 4GB....huge, night and day difference. Like a new machine.
Then I wanted more, and went from 4GB to 8GB...some difference. Not near as much noticability as 2 to 4. If I hadn't already paid for it, staying at 4 would have been fine.
But YES 8GB is fine. Few people, sellers, Apple....almost nobody will tell you this is okay. Oh yeah, and of course it voids the warranty in our 7 year old client ;).