Hardware limits, not O/S
Though I haven't searched the Microsoft documentation for theoretic limits, I haven't ever heard anyone say the operating system imposes any practical limits. Usually the practical limit is hardware - number of memory slots times memory stick capacity. The number of slots is easy enough to count just by looking, but the addressable capacity per slot may have some inherent limitations in the bit length of the memory addressing scheme, which you won't find readily in the end-user documentation. That means you have to assume it is implicit in the manufacturer's guidance of maximum memory. I've seen a number of 3-slot boards state a 3G max, but perhaps the ones you are looking at have a real 2G max (and were built with 4 slots based on the memory market being mostly 512M modules), or perhaps it is just old marketing information from back when the earth was still flat and 1G modules hadn't been invented. You might check the mobo manufacturer's website to see if they have changed the technical specs despite what you see on the retail sales page.