Time for Mr DIY to chime in....
A lil run down about me, and this kind of thing. I'm a systems analyst by trade for a large government health organisation. I basically look at a dept's requirements, choose what is the best solution based on these requirements and their budget, then blow their cash on making it happen. So you could say that for a living. I attempt to do whats 'right'.
In my non-gov-health life, I'm a I.T. consultant, I specialise in Linux (and more recently OS X too, well I'm getting there).
I've read this whole thread. I have some comments

hehe
Firstly, completely forget about windows for file serving. What a joke. Infact it's been PROVEN that Samba (on a Linux/BSD OS) serve's files faster/better than windows can.
Here's the best bit:
If you ONLY need a fileserver. Then just setup FreeNAS. I see it's been mentioned in here already. It's DAMN easy, it's got a fancy web interface, it's light, it can boot off CD or USB, or even a CF card (if you have a CF->IDE interface) and did I mention it just plain kicks arse?
I setup FreeNAS sometime ago (not current version) and couldnt get AFP working properly. But I'll tell you, if you do get it working (i must try the latest version), it will be way faster (apparently) than using samba (windows sharing).
If you need MORE than just "file serving" (say, you want web serving, an ftp server, or a web proxy, who knows)... then definately setup a Linux box.
I recommend Fedora Core or SUSE for simplicity, ease of use (even for linux gumbys) and reliability (oh and of course community support). I cant comment on Ubuntu, because I've not used it, but it seems to be pretty hot right now.
Linux is easy.
If you must know, your Mac is pretty much just like Linux. (well, BSD anyway), it has it all undernieth.
My setup:
I run three Mac's in my house. Two MBP's and one iMac (kitchen, see my sig). I have an old Intel 2.6Ghz celeron as my server, with a gig of ram and about 250gig in hdd. It runs apache for web, samba for file serving, postfix for email, mysql, php, dns services and more... It has Fedora Core 6 Linux on it. I have a 1gig net card in this machine, it's got 'okay' speed when transfering files, I wouldn't say it rocks. It's def not as fast as xfering files from MBP to MBP using AFP.
I'm about to install netatalk on it, and see what the AFP in that is like. If it's good I will ditch samba (or disable it until mates with windoze come over).
I'm seriously considering ditching this machine, and replacing it with a Mac Mini. Your probably thinking "wtf"??... my reasons:
My linux server is a bit noisy (it's in my office, and like QUIET). It consumes a fair bit of power (i spose), and it's chunky.
I can do all of the above things that my Linux server does, with a Mac mini, whilst saving space, power, and it will be much quieter. I will probably go an external firewire HDD setup on the mini for the file serving. I'll have mac -> mac filesharing, everything will be sweet.
I'm going off topic here. Lets take a step back.
Do yourself a favour and TRY FreeNAS. It's fairly easy when you suss out how to mount drives setup shares, etc.
I'm dead keen to try it out again for myself, and see how I go with AFP.
Good luck with everything (please, for crying out loud, ditch your windows idea)
