Is there a firmware upgrade for the Macbook6,1 from MB61.00C8.B00 to B02?
My 6,1 B02 has 8G RAM and works fine.  It also runs in 64 bit mode, which I enabled some time ago with the help of a simple hack (can't remember all the details, just google macbook, 64-bit, boot.efi, boot64.efi).

You could try to manually upgrade your firmware with the fw-upgrade of the macbook pro 5,5.  It has been done successfully by several people:

https://bugs.launchpad.net/ubuntu-release-notes/+bug/774089/comments/110

BTW: By doing this, you will probably also fix the MCP79 bug that makes MCP79 negotiate SATA1 with newer SATA3 drives (instead of SATA2).

System Profiler:



Note, that with 8G RAM, hibernation ("safe sleep"?) will make WindowServer crash if you're still running 10.6 (Snow Leopard).