4GB represents the maximum amount of physical memory
that can be addressed by 32 bits of addressing.
Intel has defined an architectural "addressing extention" (or PAE)
that is used by Win2K AS and Datacenter edition to allow the O/S
and applications to address past this. It does this in a manner
analogous to virtual memory paging, but at the hardware address
register level using specific hardware operations defined in the
extensions.
But other than SQL Server, there are really very very few applications
that currently will benefit at all by going past 4GB. File sharing certainly
won't notice. And there is a little context switching overhead incurred in using
the addressing extensions when you don't really need them.
If you are running SQL Server and want more than 4GB addressable
you will need to upgrade to one of the higher-end Win2K Server products.
Steve Duff, MCSE
Ergodic Systems, Inc.