Are you sure the Unibody Aluminum Macbook (Macbook 5,1) supports 6GB of RAM? I have this laptop and both the apple specs and crucial say it can only support 2x 2GB.
The hardware limit is (and has always been) 8GB, the original firmware limit was claimed at 4GB and there's apparently been a firmware update late 2009 (december 8th) which raised the EFI limit. At this point, claims become muddled, it seems you can put 8GB RAM in it but it only sees 6, unless you launch the 64b kernel (apparently)
The best summary I've found: http://blog.macsales.com/3291-putting-a-rumor-to-rest-owc-su...
Under Snow Leopard, at least, this limitation does not apply.
As far as the 15" MacBook5,1 models go – The three models released in October 2008, OWC supports to 6GB of RAM:
MacBook Pro 15″ 2.4GHz
MacBook Pro 15″ 2.53GHz model w/ExpressCard Slot (Late 2008)
MacBook Pro 15″ 2.8GHz model w/ExpressCard Slot (Late 2008)
The two models released March of 2009, OWC and Apple support up to 8GB of memory.
MacBook Pro 15″ 2.66GHz (All)
MacBook Pro 15″ 2.93GHz (All)Like so:
[[ Model Name: MacBook Model Identifier: MacBook5,1 Processor Name: Intel Core 2 Duo Processor Speed: 2.4 GHz Number Of Processors: 1 Total Number Of Cores: 2 L2 Cache: 3 MB Memory: 8 GB Bus Speed: 1.07 GHz ]]