How to enable more than 4GB RAM support on Linux 32bit servers?

# uname -r
2.6.18-8.1.3.el5

# yum info kernel-PAE

Available Packages
Name   : kernel-PAE
Summary: The Linux kernel compiled for PAE capable machines.

Description:
This package includes a version of the Linux kernel with support for up to
64GB of high memory. It requires a CPU with Physical Address Extensions (PAE).

The non-PAE kernel can only address up to 4GB of memory.
Install the kernel-PAE package if your machine has more than 4GB of memory.
# yum install kernel-PAE

Q:how do you enable the PAE kernel after you reboot?
A:Just install kernel-PAE and boot it. This kernel is PAE-enabled.

Then ensure the followings:
#cat /etc/sysconfig/kernel
# UPDATEDEFAULT specifies if new-kernel-pkg should make
# new kernels the default
UPDATEDEFAULT=yes

# DEFAULTKERNEL specifies the default kernel package type
DEFAULTKERNEL=kernel-PAE

After the server reboot PAE kernel will be enabled and it will automatically detect/handle more than 4GB of RAM.