http://msdn.microsoft.com/library/en-us/memory/base/physical_address_extension.asp?
Typically, a process can access up to 2 GB of memory address space
(assuming the /3GB switch was not used), with some of the memory being
physical memory and some being virtual memory. The memory manager uses
PAE to provide more physical memory to an application, which reduces
the need to swap memory to the page file and increases performance. The
application itself is not aware of the actual memory size. All of the
memory management and allocation of the PAE memory is handled by the
memory manager independently of the application.
Applications that are 4GT-aware are likely to remain in physical memory
rather than be paged out, which increases their performance. The
exception is when the /3GB switch is used in conjunction with the /PAE
switch. In this case, the operating system does not use any memory in
excess of 16 GB. Therefore, if the system restarts with the /3GB entry
in the Boot.ini file, and the system has more than 16 GB of physical
memory, the additional physical random access memory (RAM) is not used
by the operating system. Restarting the computer without the /3GB
switch enables the use of all of the physical memory.