Debian is by far the best distro for a minimalist server on older hardware. The apt-get is great, and the install is a breeze. However if you've been nothing but a windows user for all your life with no command line or text-only apps like fdisk to back you up, installing debian can be intimidating.
Going from a OS which restricts you to cd-only installs with a standardized click and drool interface, to a distro that actually wants you to download it over the internet as the perfered way of installing it can be a bit of a culture shock to say the least and is fairly frustrating experiance. 
Any sort of Linux distro would be great for this sort of thing, so if you don't want to let go of a familar interface go ahead and install anything you want. Just keep in mind that X windows is a dog on slower hardware, and you don't want to use it unless you realy need to. I usually don't even keep a monitor or keyboard attacted to my servers. I just do everything using ssh over the network, and if you only use windows on your desktop you can use a program called putty.exe as a ssh client. That's a great way to learn linux.
As far as rpm's vs apt-get. Rpm's are a mess, unless you use redhat-only sources of rpms you can get into trouble...
Anohter thing, A router firewall usually shouldn't be used as a server, it's just a security gotcha. basicly any pc router/firewall should be kept to a bare minimum, just enough to do it's job. The more services you have the more vunerable, but as long as you know what your doing you'll be find... Just remember running services like ftp, telnet, ssh, and/or p2p network stuff is gonna make you more of a target then the run-of-the-mill internet connection. Kinda like stupid people who thump lound stereos systems while driving around there own neighborhood, makes your system more attractive crackers and theives (and they probably know were you live)...
Any thing above a 486 computer will make a great file server or firewall/router. Hell, my dad's office used a 486dx prolient compaq server as a file sharing server for 50-60 people just untill a year or so ago... (then I got it). A 200mx apache webserver running static content can easily saturate a 10MB and maybe even a 100MB ethernet network with network activity, and a T1 line only goes 1.33Mb's or so.(providing active content like these forums is a entirely different can of worms though....)  I use a 200MX for a router on my cable line with five or so computers behind it's firewall, and it runs fine for 2 years now with no problems.