According to
this page it has the server packages. As you're familiar with it, it's the obvious choice.
I run Slackware servers (just my preference). The one at home runs on an AMD K6-III/450MHz with 128MB memory.
If you want to use Ubuntu for a business server, stick to a
Long
Term
Support version (8.04 currently). 8.04 is supported till april 2013 while 9.04 will only be supported till october 2010.
Personally I will never go for Ubuntu server as the default install lacks some stuff like the
ssh daemon. So that implies extra downloads afterwards. I installed it once, found that out and threw it off again.
Just on a site note:
You don't want to waste processor power and memory on a graphical user interface. So Ubuntu server editions come without the graphical environment (can be installed afterwards). So if you're not familiar with command line, it's time to get the experience.