Getting 32-bit Windows to recognize anything beyond 2GB of installed RAM is a seemingly futile effort. Even if your hardware supports the possibility, the software may hold you back - and performance gains are questionable:

The reduction in available system memory depends on the devices that are installed in the computer. However, to avoid potential driver compatibility issues, the 32-bit versions of Windows Vista limit the total available memory to 3.12 GB. If a computer has many installed devices, the available memory may be reduced to 3 GB or less. However, the maximum memory available in 32-bit versions of Windows Vista is typically 3.12 GB.

So, people use a PAE switch to get past the imposed 2GB barrier. Here’s a description of the 4 GB RAM Tuning feature and the Physical Address Extension switch:

When the /3GB switch is used with Windows XP Professional, with Windows Server 2003, Datacenter Edition, with Windows Server 2003, Enterprise Edition, the /3GB switch works identically across versions. This functionality lets device-driver developers test their drivers in this configuration without having to install one of the Windows Server 2003 products just listed. The user-mode memory space is now limited to 3 GB.

Sounds good, right? Not so fast:

The /3GB switch can cause some applications to have problems that are related to address dependencies or to a reduction in kernel space.

Bottom line: if you have 4GB of RAM in your system (or more), and you want to take full advantage of it, start using a 64-bit OS.