Pen-Testing New Server - Where to start?

[Security <security () gridrunners com>]

Hi, I'm new to the InfoSec industry and would like to try my hand at 
penetration-testing (and securing) a new server I've set up at home.

Seeing as I've set up the system, I know all the usernames/passwords 
used on the box, as well as how everything is set up, but I'd like to 
approach this as an outside user, pretending that I have none of this 
information. I want to try to gather information, form an attack plan, 
and attempt to crack the system from scratch, so that I can later on go 
back and secure the system against those attacks.

Here's the information I can assume I'd know, from basic enumeration:

The server is running Ubuntu v6.06, with the following services:
ftp
http (apache)
smtp
pop3
irc (hybrid)
ssh

When setting up the system, I followed the following tutorial (almost to 
a T... though I did a few things different):

http://www.howtoforge.com/perfect_setup_ubuntu_6.06

Since the system is on my local network, I know there's only one IP I've 
got to worry about, and this is the only target machine.

Any ideas where I should start? What information might help?

Thanks.

~Xor