One of the biggest issues when running RedHat Linux in any verison is how to get updates when you do not pay for mantenance. As far as I know, you cannot get the updates from redhat for free once your 30 day trail is over. Many companies need to run redhat due to the applications we designed around redhat in linux. There are many ways to get around this when running different flavours of linux, but they necessary become a pain in the backside when it comes to this, as the next update of the flavour may break the application. A workaround is too use the free version of redhat enterprise linux which is CentOS. You can convert a redhat machine to CentOS, very easily. Below will show you the process of how it is done.
At the time of this post CentOS 5.8 is the highest release to upgrade a redhat enterprise 5 installation. I am yet to work out howto upgrade a redhat enterprise 6 release to CentOS 6.