There is a lot of noise about amount of memory Mac's support, so I decided to try and have installed 4GB stick in my 2007 2.2GHz SantaRosa Macbook 3.1 , and it voila! it works.
And yes, it works even when active memory crosses the 4GB line.
Now the machine never uses swap and that helps a lot, especially when I work with bunch of Java applications and GigaSpaces pieces.
Hardware Overview:
Model Name: MacBook
Model Identifier: MacBook3,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.2 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 6 GB
Bus Speed: 800 MHz
Boot ROM Version: MB31.008E.B02
SMC Version: 1.24f2