CentOS itself is just a distribution of Linux, so it doesn't have an interface.
From my understanding of it, CentOS is like RedHat (which favours reliability a little more than new features, and costs money as it is supported) and is free.
Webmin is an open-source package that adds a web-based control panel for many of the common functions like Apache web server, databases, scheduled jobs and package installation.
VirtualMin goes on top of WebMin and sets up web page domains, email (although I certainly recommend hosting your email elsewhere instead of using your valuable server resources and time).
If you haven't run your own server before, I would definately stick with Apache. With a bit of caching here and there, 6000 visitors should be fine.
Virtualmin screenshots:
http://www.virtualmin.com/screenshots
Webmin screenshots:
http://www.webmin.com/demo.html
If you have a spare domain name you don't use, I recommend getting a Linode account (you can get a refund if you don't like it), trying to set it up, and see how you go.
It is a far better choice than Dreamhost VPS which I notice went down for many hours last month
