Today I needed to make a CentOS 6 32bit OS see 24 GB RAM. Unfortunately, although the default 32bit kernel from RH already has PAE enabled, it will not handle more than 16 GB RAM, the only solution that came to mind was to use a 64bit kernel.
This is possible, but does not seem like a very good or elegant solution, at least not for long term; however it's WAY quicker than a full reinstall.
All one needs to do is get the 64 bit kernel from a mirror and install it via rpm:
rpm -ivh --force --ignorearch kernel-2.6.32-431.el6.x86_64.rpm
That's it, now edit /boot/grub/menu.lst to make it default and off you go: reboot!