Which model? 2004 model I think can only address 8GB, the 2005 model can address 16GB.

edit:

"There are two versions of this model (ML/9216A), which shipped from January 6, 2004 to January 4, 2005 with 512 MB of RAM and a CD-ROM drive, and M9743LL/A, which shipped from January 4, 2005 to August 7, 2006 with 1 GB of RAM, a DVD-ROM/CD-RW "Combo" drive, and a maximum RAM capacity of 16 GB (up from 8 GB in the first configuration)."