If you have an asus p2b rev f laying around, with a p2 or p3 450 or better, can't go wrong there. There are literally dozens of good boards tho, just suggesting something that may be laying around unused, but truly stable as can be.
For just local fileserving on a small LAN, 400~500 cpu's and 256 of ram will do it in win2k3 or win2k.
I would go freeBSD and Samba on anything better than P166, and sell that 2k3 for beer money, but that is just me
