Actually, the amount of RAM OS X can handle is limited by the hardware, not by the software. It is natively 64-bit and 32-bit, so unlike Windows there aren't separate versions for each type. Thus, it can handle huge amounts of RAM. Right now, I think you can pack 32GB into the Xserve, Apple's server option, and 4GB into the MacBook Pro. The MacBook Pro is limited in expandability by Intel's chipset.
OS X is a memory wizard when you have limited RAM and when applications require lots of RAM. For example, I think that Jo did a test to see how much memory TG2 could address before it crashed, and found that it can access up to 3.5GB at a time on OS X, while only being able to access 2 or 2.5GB on Windows.
*Takes off Apple hat that he hadn't even realized he had put on
*