Mine is either the 3,1 or the 4,1 model. It can most definitely handle the amount of RAM I want to put in it. No question. Plus, with 6 GB RAM, when the GPU grabs RAM for itself, I still have over 4 GB RAM for the system to use, which is what I would have if it was a discrete card and I had the official max RAM in.
I'm not sure which one it is, because when I looked up the serial on Apple, it said it was the 3,1. But when I actually took it apart, it had the Penrynn layout on the logic board, which is the 4,1 model. Of course I'm hoping it's the 4,1 model, but that one came with a 250 GB drive and mine has a 160 GB in it. So I'm assuming it's the 3,1 model, but it's strange that the logic board had the Penrynn layout.
It was a refurb unit, so that may explain it.
@lokster: If you have the 1,1 model MacBook, it was introduced in May of 2006, so it's closer to 4.5 years old than 3. Mine is right about 3 years old...