How can I identify the chipset of an (old) MacBook?

The first MacBook to fully support 4GB of RAM (with unofficial support for up to 6GB) was the MacBook (13-inch Late 2007). This was also the first MacBook to feature a 800MHz FSB and the GMA X3100 graphics chip.

I'd be interested to know where you're getting your system specs from, but I'd suspect that somewhere along the line the FSB and RAM speed are getting confused.

Check your system's specification from the System Profiler application found in /Applications/Utilities. Under the Hardware section look at the "Model Identifier".

If you get MacBook3,1 then you're in luck - that's the newer (Late 2007) model with greater RAM support and graphics. Anything prior to that (Macbook2,1 or MacBook1,1) and you're looking at (approx) 3GB limit or even 2GB (for the very first gen model).