the 6GB limit on the MBP/MB3,1/4,1 and iM7,1/8,1 is down to a Memory map issue with the i965PM/965GM chipset/EFI Firmware
you CAN install 8GB of RAM in those systems however they will either fail to boot OS X properly, or while they may boot, if you use more then 6GB of RAM the system falls over
MCP79 macs dont have this bug, and as such, 8GB works fine in those systems (poking around in the firmware of the MB5,2 shows an actual theoretical limit of 32GB, but good luck getting there on DDR2 SODIMMs

)
what people dont realise with the MB5,2, is that motherboard architecture wise the MacBook5,2 is completely different to past Plastic MacBooks, and the MB5,2 shares MUCH more in common with UniBody MacBooks then it does with any Pre unibody otherwise (the MB5,2 is very much a wolf in sheeps clothing)