A gui is not necessary for getting madwifi working, nor is it needed for a server.
First, open up in a text editor: /etc/apt/sources.list and be sure that you have at least main and restricted repositories. Look for a line that looks like this:
deb http://us.archive.ubuntu.com/ubunutu main restricted
(note, if you live nutside the United states, the beginning of the line will have something other than 'us', e.g. http://de.archive.ubuntu.com
sudo apt-get update
Ubuntu has madwifi drivers in the linux-restricted-drivers package, check to see if you have that installed:
dpkg -l 'linux-restricted-modules*'
if they are not installed, install them with the command:
sudo apt-get install linux-restricted-modules-$(uname -r)
then active the madwifi module:
sudo modprobe ath_pci
to use the interface, you need the wireless tools package:
sudo apt-get install wireless-tools
check to see that the interface is 'there':
if you see ath0 and a lengthy paragraph, it is there.
associate the interface with an access point:
iwconfig eth0 essid "your_essid"
(replace "your_essid" with name of the essid that you want to associate with, e.g. if you have set the access point in your router with the name george, it would be: iwconfig eth0 essid "george")
assuming that you will be using dhcp, get an ip:
This should get you started, for further reading, particularly if you are using wep encryption, check out the man pages of iwconfig (man iwconfig). Doing a google search of wireless-tools, madwifi ubuntu, will give you more information.