I was thinking of upgrading my 2007 MacBook Pro with more RAM. It came with 2GB, and the specs say it can take up to 3GB, although some online sources say they can successfully install 4GB. Apparently, these older MacBooks map "system functions", I guess meaning IO mapping and ROM into the region between 3GB and 4GB.
... at least 3 GB of RAM should be fully accessible, while when 4 GB of RAM installed, ~700 MB of of the RAM is overlapping critical system functions, making it non-addressable by the system.
OK, so no 4GB for me, but what about replacing one of the 1GB sticks with a 2GB stick for a total of 3GB? It turns out that if I did that, I'd take a small performance hit.
All Intel Core Macs support dual channel memory access if matching modules are installed. The customary estimate is that this gives a 6% - 8% real world performance benefit. The modules do not have to be the same brand. That means it is quite possible but not 100% guaranteed, that adding a 3rd party SODIMM to an Apple supplied SODIMM of the same size will make a matched pair.
Verdict: don't bother...