No one wish to reply am stuck on this one?
To be honest, most low range motherboards & processor will be more than enough to run a basic file server (ie file shares/mapped drives) as the amount of work needed to service the data is fairly low.
I have even used an ARM (AM11 processor I recall, a lot less powerful than most phones) card running linux with 512MB ram and 6 USB drives over 2 powered usb hubs serving 3 users watching vids with no problems which kinda shows just how little cpu is required for basic file sharing.
A low end i3, even a "pentium", would probably be more than powerful enough for basic file sharing. If you do anything more such as a DB (database) server or mail server application server (as in applications run on the server, not programs are loaded from a share and run on the local pc's) and so on... then a little more power might not go amiss.
It all depends on what you want to do with it, how many users, how critical/recoverable the data is, how much up time you need, software/hardware RAID. and so on... If you're running it as part of a business and not a home user network, that's a whole different ball game.
My current i7 4790k pc also doubles as a home network server (3 users) runs linux with mdadm software raid6... anecdotally, serving file shares to 3 users watching 700MB mpeg vidieo files concurrently uses less than 1% cpu of _1_ core. smbd (samba the file share software) currently is showing .3% cpu of a single core with one user attached, which is 0.075% of all 4 cores.
