My father has a small law office (him, his brother who is also a lawyer, and about four secretaries/paralegals) and wants to have the documents on the various office computers backed up; I'm looking into setting up a simple system that will take all significant documents off the computers and make, say, nightly backups of new and changed files.

I've already been pointed to rsync and rdiff-backup as software to get and save the files; what I'm asking here is, since I'm building the system, how powerful does it really have to be? Can I use something like a KuroBox (266MHz PPC, 128MB RAM) or an XBox, or do I need something more up-scale? I know essentially nothing about servers bud don't see this as an overly complex project.

Also, other than the above software and Samba, what software might I need? The setup of the office network is all the computers running various versions of Windows, with, as far as I know, Win95, Win98+, and WinXP currently present on the network. I honestly don't know how the things are connected to each other, as the "router" is a telco box of some kind that's been physically affixed to the wall for over ten years; I do know that the secretaries are currently using Windows "Shared Folders" without problems.

I've got plenty of time to spend learning how to set this up software-wise; all I need is suggestions on the hardware, any inobvious applications, and tips from anyone who's done something similar in the past.

You don't need powerful hardware. Something like this might work: http://www.newegg.com/Product/Produc...82E16856115030

Not bad for only $228 + $8 shipping (including AMD Sempron 2600+ CPU - should be plenty of CPU for your needs). Add 512Mb memory (about $40), a DVD burner (about $40), and a harddisk (cost will vary with size, but 250Gb can probably be had for around $50 if you wait for sales are are willing to put up with mail-in rebates). Share a spare keyboard / mouse / monitor for initial setup and then give them back to their original owners and use SSH to access this computer. Sound, graphics, and network are integrated into the motherboard.

For simple backups (only), you might be able to get by with something like this:
http://www.newegg.com/Product/Produc...82E16822144385 or http://www.newegg.com/Product/Produc...82E16822148076 or http://www.newegg.com/Product/Produc...82E16822144023

I don't know if any of those three external harddrives are any good - I just picked them at random to illustrate some of the different capacities available. Note however, that the external harddrive option would be more manual - you'd probably have to move the thing from system to system to backup. A sure-fire human failure point just waiting to happen. I'd personally spend a bit more on a real Linux server that will do all this unattended, like you were initially planning. Just suggesting these things as an option, if your dad and company decides to go cheap on you!

Building on a NewEgg barebone was my /other/ plan. Sounds like the way to go.

Also, I'm going to weep now, as that processor is what I'm using on my currrent box (or a step up).

As far as cheap, I will be more concerned with that than my father, as he wouldn't blink at a $500 setup - not that I see any reason for such a thing.