Yea, the thing is DDR2 is only ever going to go up in price now it has essentially started to become obsolete. It depends really on how you ever intend to use your Mac but, regardless, if you never intend on even going up to 16GB then just get 2GB DIMMs to Apple's specification, that way you can't go wrong on any count can you?
Larger modules like 4GB single modules were only ever intended for specific scenarios anyway like servers, or for people with large amounts of dosh to blow, and the faster the memory becomes (such as DDR3) the more feasible larger module sizes become at lower prices. Apple's specs for your Mac may well state it as only holding upto 16GB RAM but they could either be directly referencing that they've built the memory controller to not recognise anything beyond that amount, or, they're just referencing the fact your average end-user wasn't ever likely to use modules larger than 2GB in size in any one slot.
I don't think anyone has ever tried to put more than 16GB in their G5 Quad, and if they have they haven't bothered to advertise the fact for some reason, so it would seem no one could answer your question reliably. Besides, 16GB is a huge amount of RAM even by today's standards, and it's highly unlikely you'd need even half of that in any given situation you may be going to use your Mac for, so in reality it'd just be easier and more cost effective for you to adhere to Apple's specifications and buy PC2-4200 2GB modules.