First of all: If it's a production server. Don't do it. Installing a GUI will slow performance and may open more security holes. Also never develop directly on a production server.

Secondarily: If anyone sees this prior to installing Ubuntu Server, you should probably install the desktop version of Ubuntu instead and then install the server-services, since that's quite a lot more well documented and you're probably not going to run Eclipse on a dedicated (production-)server anyway.

And now the answer:

First install the ubuntu-desktop package with # apt-get install ubuntu-desktop (do this as root, or by prepending sudo to the command), then reboot your computer and install eclipse as you would on any other Ubuntu computer.

See section one on http://www.ubuntugeek.com/install-gui-in-ubuntu-server.html for more detailed instructions