Can I upgrade my MacBook with more than 4GB of RAM? I have an Early 2008 13 inch MacBook updated to Lion. According to this Apple KB the max RAM I can add is 4GB. 

I know the OS that came with the initial computer didn't support 64 bit. So is this 4 GB limit a hardware or software limitation? Can I add more than 4 GB now that I have Lion (I understand it supports 64 bit)?
You can add up to 6GB of RAM in your MacBook. You can see the list here.