for that matter, for a simple file server, would SATA drives even be necessary? I just figured it would be a little easier to fit multiple drives in there if they were SATA drives and controllers.

If you want more than a couple of drives in there, SATA is the way to go. However, unless you need to put lots of drives in there right now, you'll probably never need to: if you're like me you'll buy a new drive every three years or so, all of which will cost the same amount and each of which is so much bigger than the previous one that it's not worth running the previous one alongside it.

And what are the requirements for a computer that will be doing next to nothing other than serving files to another machine? How much memory do I need? For that matter, do I even need an OS? Is there a way to set it up without one?

I use a Pentium MMX 233 with 128Mb, which is much more than adequate. That's for Linux, though; you'd likely need more if you attempted to do this using Windows.

Peter