RHEL / CentOS Support 4GB or more RAM ( memory )

Run command:
# uname -r
2.6.18-8.1.3.el5
Run command:
# 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
Loading “installonlyn” plugin
Setting up Install Process
Setting up repositories
Reading repository metadata in from local files
Parsing package install arguments
Resolving Dependencies
–> Populating transaction set with selected packages. Please wait.
—> Downloading header for kernel-PAE to pack into transaction set.
kernel-PAE-2.6.18-8.1.15. 100% |=========================| 207 kB 00:00
—> Package kernel-PAE.i686 0:2.6.18-8.1.15.el5 set to be installed
–> Running transaction check

Dependencies Resolved

================================================== ===========================
Package Arch Version Repository Size
================================================== ===========================
Installing:
kernel-PAE i686 2.6.18-8.1.15.el5 updates 12 M

Transaction Summary
================================================== ===========================
Install 1 Package(s)
Update 0 Package(s)
Remove 0 Package(s)

Total download size: 12 M
Is this ok [y/N]: y
Downloading Packages:
(1/1): kernel-PAE-2.6.18- 100% |=========================| 12 MB 00:12
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
Installing: kernel-PAE ######################### [1/1]

Installed: kernel-PAE.i686 0:2.6.18-8.1.15.el5
Complete!

Note
After install kernel PA. We config file /etc/grub.conf
#vi /etc/grub.conf
# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,0)
# kernel /vmlinuz-version ro root=/dev/sda3
# initrd /initrd-version.img
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-128.1.16.el5PAE)
root (hd0,0)
kernel /vmlinuz-2.6.18-128.1.16.el5PAE ro root=LABEL=/ rhgb quiet
initrd /initrd-2.6.18-128.1.16.el5PAE.img
title CentOS (2.6.18-128.1.14.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-128.1.14.el5 ro root=LABEL=/ rhgb quiet
initrd /initrd-2.6.18-128.1.14.el5.img
title CentOS (2.6.18-53.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-53.el5 ro root=LABEL=/ rhgb quiet
initrd /initrd-2.6.18-53.el5.img