To support more than 4 GB memory

By default the kernel's scope is capped at 4GB memory, it won't see more than this amount even if memory is physically there.

To extend kernel's sightseeing, the Kernel PAE package is needed, which provides support for up to 64GB of high memory. A CPU with Physical Address Extensions (PAE) is required, most modern CPUs support it so no big deal.

# yum install kernel-PAE
...

# init 0 (of course a reboot is needed)