Some motherboards are limited by the chipset. I recall some... Conroe-based Macbooks? that had a hard limit of 3 GB based on the Intel chipset used. Then there's the wonderful bang-for-buck Dell Inspiron 410. It officially supported 4 GB but after a BIOS update it started to support up to 8 GB. But wait, there's more! Most OEMs come with Windows Home Premium (let's go with Win7 for this example) which supports only 16 GB:http://en.wikipedia.org/wiki/Windows_7_ ... ison_chart
So from a practical standpoint, even if your motherboard supported 64 GB of RAM, if you're rocking Win7 Home Premium, you'll only get use of 16 GB of that.
This is a question I myself am curious about, too, as I have a Dell XPS 8300 and the max supported RAM is supposedly 16 GB. I'm probably just going to buy 32 GB of RAM just to see what happens sometime as no one else seems to have done this. Is it limited in the chipset? In the BIOS? By Dell assuming a lowest common denominator of Home Premium and not wanting to be sued for claiming more RAM than it lets the customer see? Who knows!