You're interpreting the page wrong.

Physical memory=RAM=4GB max that can be accessed by the OS. So if you had 6GB of RAM, for Regular Windows Vista, it would only use 4GB, the /3GB switch doesn't have anything to do with RAM usage there. For Vista Datacenter edition it has to do with RAM usage, too.

Still, the 3GB switch (or "bcdedit /set IncreaseUserVa 3072" on Vista) changes the way a process and windows share the 4GB of address space (that has absolutely nothing to do with RAM).

You may have 10 processes with 2+2GB address space or with /3GB switch 10 processed with 3+1GB running, providing there's enough virtual memory (via RAM or swap file) running at the same time.

Without the /3GB switch, Poser will only use up to 2 GB, Windows reserves the other 2. With /3GB switch, Poser can use up to 3GB and Windows only reserves 1GB.