Is it possible to have files hosted on a windows computer that could then be FTPd over to a Mac's Lion server, and then to clients? The windows is pretty capable hardware-wise, as is the mac, I'm just wondering if I need any extra software or how I can find out how to do the setup.
Yes, no shortage of both free and commercial FTP servers for Windows. See http://en.wikipedia.org/wiki/List_of_FTP_server_software
But if the goal is simply file sharing - you may want to consider simply enabling Windows file sharing and then configuring your Mac to connect to the Windows share.