If you're going to be sharing files with Windows machines, you'll want to learn the ins and outs of SAMBA, which is the *nix software that shares file and printers with Windows. If you go with Debian and you use the stable version, I'd suggest going with the 2.6 kernel (2.4 is the default on stable) since I've found it to be a bit snappier and more likely to work wirh newer hardware. You might want to look at www.openfiler.com
which is CentOS optimized for file server duties (CentOS is itself a rebuild of Red hat Enterprise Linux).
In any case, you have a lot of learning to do, so good luck! One more piece of advice -- check that your hardware is supported by Linux (most common stuff is with the exception of some wireless cards and winmodems) before
you shell out $$$ for it.