I Hope it helps I will tell you how my network is set up. I have four of my own pc's all with internet and I work on other peoples pc's as well. My main router is upstairs beside my modem and main computer with an extra work station and KVM. My wife and kids have their own computer in a room, and downstairs in my workshop I have my server and a system I can do basic computer tasks on and another workstation. I use simple Network Neiborhood file sharing and it works very well. You don't have to But I map the servers hdd to a network drive on all the other machines (just so it shows up in my computer as the Z:/ Hard disk. That is the only way I can make my wife and kid's use it. The problem with only using one router is only having four ethernet connections so I used a cheap POS wireless router as a dumb switch and hotspot. now I have 7 wired ports and still have the wireless. You can just hook the wan port on the wireless to a lan port on the main router but it causes conflicks with file sharing. If you want to know more then Here's some more info on that
I use nero 6 backup and I love it. It has auto back up and such and is simple to do.
If you know linux it is also an option and if you have means of obtaining windows server 2000 or 2003 then that would be better as well. Although with VNC and KVM switch's I never have to leave my desk, lol I hope this help's
Oh and being behind a nat router is the best firewall in the world but if you want anti virus why not use AVG its free and works awesome.
Edit - You don't need alot of ram for a file server 256mb is plenty If you are just going to use XP as the server that is.
Also if you map the hdd on the machine you use it just like it was on that machine so you can just creat your files on the mapped drive and it will always be there.
And why would you want the os on all four hdd's??