Since this is a learning experience, here are some links,
Smartdraw has a 30 day free trial, is downloadable, and is used for network design diagrams.
Good network design examples from Lebanon of all places. Cisco has some good design principles, pay attention to the 80/20 rule.

Microsoft
has some good reference material.

Remember, the best security setups have one achilles heel- the users. It used to be p2p was opening backdoors but now it's IRC and IM. Also, file downloads of any type can set up trojans, etc. I'm not sure if it would be effective but spyblaster or spyware search and destroy set to run at bootup (and restricted to administrator configuration) would help a lot. We thought we had pretty well secured our network from the blaster worm/virus but someone with a laptop using vpn loaded it into the network. So be pretty careful of vpn, its handy but does have a cost. Especially overtime when the techs have to scan servers, etc.