Why are you paying for "CentOS?" Does Rackspace charge for different distributions? If you're going to pay for a distro then might as well go for the source and pay for RHEL.
I'm basing this from your comment: "CentOS 5.4 - pretty much an exact replica of RedHat 5.4, so there's lot's of online documentation and help. Plus, it came as an inexpensive option with Rackspace. :)"