I would use Centos, its a clone of Red Hat RHEL. And the best part its free. Centos 4.2 is going to be supported until 2012. Not only that there is a ton of community support for Centos.
www.centos.org
Personally I use Centos 4.2 server on my server. Its rock solid and stable. I haven't had any trouble with it. I have been using Centos for over a year now. I will be using Centos on every server I build from now on.