With two 2GB chips, the computer might recognize just 3GB.
The computer recognizes 3.5GB is what I understand, or else why bother and just use a 2GB + 1GB right?
I think the article is trying to say is there is a problem with memory overlap when one uses 2 - 2GB for 3.5GB and that they would be better off using 2GB + 1GB for 3GB to avoid the overlap.
However one would lost the .5GB and have to swap slightly more often to storage.
The older 32 bit Mac's, if you have a 7,200 RM drive (or splurge for a small SSD) then the swap is faster, so 3GB + a fast storage would likely be better than having memory overlap with 3.5GB.
I replaced my older 15" with a 7,200 RPM and 4GB of RAM and the performance improved greatly, I really didn't have any problems until I install OS X Lion on it and that killed it deader than a doornail.