Ubuntu 10.04 Server Edition will be fully maintained through April 2015, there is no rush to update:
https://wiki.ubuntu.com/Releases
You can easily update your 10.04 directly to 12.04 if you wish. However, if you do so, it is prudent to make a complete backup, test that your sites & apps & services will work correctly with the updated system, and (if your server is mission/business critical) thoroughly test the upgrades on your test machine before applying them to your production server.
CentOS is very nice too, I'm not saying anything bad about it, just that if you are currently happy with Ubuntu, there is no overwhelming reason to switch.
(ps I do not recommend Gnome/Xfce for a server; any sys admin who replaces you in the future will know command line, it is a basic linux admin 101 skill.)