Since I had a hard time finding this, I’ll post it here in the hopes of helping others out. Older Core Duo MacBook Pro laptops can only address 2GB of RAM. Many people will try to tell you differently, perhaps not understanding exactly what’s going on. A Core 2 Duo will see plenty, and I think newer MacBook Pros will see beyond 2GB, because I’m pretty sure I read about someone who got 6GB in one.

But a 2006-era Core Duo (not Core 2 Duo) machine will not see more than 2GB RAM. If you’re cockily sure that this is in error and buy a 2GB stick to replace one of the 1GB sticks, you’ll have a laptop with 3GB RAM, but it won’t boot. Trust me on this.

The good news is that my Thinkpad takes the exact same memory. (The bad news is that I rarely use it…)