Ubuntu 8.04 currently has fglrx 8.3 in the repos, though it doesn't include the full Catalyst packages. The repo driver works fine but if you want to stay current with the ATI driver releases (and use the full Catalyst control panel), you shouldn't count on the Ubuntu devs to package them for you. You'd be holding your breath until next October, because they don't like to update ("backport") drivers to add features... they usually only backport to fix serious bugs.
So... you have to compile your own. Using the ATI script is easy and pretty foolproof if you follow the directions at http://wiki.cchtml.com/index.php/Ubuntu
I hope this helps! Best of luck to you.