Licensed Memory in Windows Vista
http://www.geoffchappell.com/viewer.htm?doc=notes/windows/license/memory.htm

Though machines with 4GB are not yet the typical purchase for home or business use, they are readily available from major manufacturers and it won’t be long before they are the typical purchase. But there are problems. You don’t have to stand for long in a computer shop to hear a sales assistant talk of 4GB as some sort of limit for 32-bit operating systems, and it won’t be long before this sales patter develops into outright promotion of 64-bit Windows as the only way to get past this limit. Some sense of this can be seen already in manufacturers’ advertising materials, as in the following fine print from Dell:

You probably already know this, but this page describes PAE and how a 32-bit OS can use more than 4GB of RAM (and already does - see Windows Server editions).