you can only get qos on the server versions of windows, but if you do decide to use xp in the end, you will be able to do routing with it, as well as use it as a game server.
then as far a splitting the internet, thats what the router does. if you are going to use windows xp, you just use ICS (internet connection sharing) and it does mostly everything for you. with the server versions, there is a more advanced and powerful version of ICS used for routing. on the hardware side, you just plug the modem into one ethernet card, and the switch that all the other computers go on in the other.
as far as two IP's go, that is not two internet connections, that just means you can hook up two computer that are globally addressable before they want more money. i wouldnt even bother with that. if you use the computers inside a NAT protected network, more likely than not youll avoid most of the viruses and such out there. just make sure to keep your server well defended by setting up port rules and such.
as far as building a new machine goes, it really depends on how many people are going to play. on my game server it has a 733 mhz celeron and 394 ram, and it will handle 8 people easily with ut2k3. i dont believe that you can support more than that without lag on a broadband connection like you are going to have.