Apple has consistently published specifications for machines that state limitations on how much RAM the machine can take. In some cases, experimentation has found that certain Macs can take more than the ‘official’ amount of RAM. iMac, MacBook and MacBook Pro models from 2008 have long been able to be used with 6 GB of RAM (a 2 GB module and a 4 GB module), but when installed with 8 GB they crash as soon as the RAM above 6 GB is utilized.
It seems that Apple released some EFI firmware updates which in combination with OSX 10.6 Snow Leopard allow certain machines to go to 8 GB total RAM (2 x 4 GB SODIMMs)
The machines affected are MacBook and MacBook Pro models from late 2008. Their Mac Identifier numbers are MacBookPro5,1 MacBook5,1
In order to use 8 GB of RAM, the machines have to be at the latest Firmware ROM version (below) which should have been made available through the Software Update process. You can check your ROM version by going into the Apple Menu; About this Mac; More Information; Hardware Overview and look for this line
Boot ROM Version: MB51.007D.B03
Checking the Boot ROM version http://support.apple.com/kb/HT1237
For a MacBookPro5,1 the version needs to be MBP51.007E.B05
For a MacBook5,1 the version needs to be MB51.007D.B03
If yours isn’t up to date, back up your data files (just in case) and then run the EFI updater from here
MacBook Pro http://support.apple.com/kb/DL975
MacBook http://support.apple.com/kb/DL974
Do not use these updaters if you have any other model of Mac or version of MacBook.
After the ROM is updated you should be able to upgrade to 8 GB of RAM for maximum performance.