Pretty much all routers on the market can handle 4 directly wired computers (or more if a switch is added), and a huge number of wireless (256 for most iirc).
So if you are using a second router to connect the wired pc and xbox360, you should be able to run a wire from that to the main router and connect to the 4th free port (as the other 3 will be taken up by the xbox and 2 PCs).
I assume from your post that the router you already have is wireless. If that is the case it will allow you to do a number of things:
1. Buy a wired router, and use the existing router for all wireless connections, and wire the existing router to it using ethernet cable.
2. If the existing router is a cable router, then you could just buy a switch to allow more than 4 pcs/consoles to be connected wired.
3. Buy a new wireless router. This way you can place both the new router and the existing router spearately (eg ground and second floors), with one set as DHCP/DNS and the other as a slave access point, allowing you to maximise your wireless network coverage. You can then connect these together either wired or wireless.
As for traffic shaping, the cheapest way to do this would be to use a software shaper, rather than have it built into the hardware (as to my knowledge this would be out of your budget)