1. First, this is a router, not a modem. That becomes important later on. As for "more speed", the router is no magic fix. It will not give any more speed then that is going into your modem by your ISP, and probably less then. It can be help with traffic priority, but that is more specific to the upload and not the download. It can give priority, but if all your devices are on and your total bandwidth consumption is greater then what your ISP can provide, there will be lag. QoS helps, sometimes alot, but it isn't 100%.
2. Don't think of it so much as priority for certain devices, but certain traffic types (protocol and port) that are coming out of those devices. For instance, you can say give traffic used for multiplayer on a PS3(UDP on port 5223) priority over web traffic for your computer (TCP on 80) but if you set your PS3 to First priority and 360 on 2nd priority, their really isn't going to me much of a difference. Its a scale of 1-255. So a 1 digit difference is almost identical.
3. Yes, as long as they are using different ports, you can distinguish them. If you have two 360's or PS3, you need to use UPnP, since the router won't know the difference between the traffic coming from both of them.
4. Yes, you can set QoS(AkA Gamefuel) on wireless devices. All Wireless (WLAN) traffic goes through the network (LAN) to get to the internet (WAN), so if you set prioties on traffic from LAN to WAN, it will effect wired and wireless equally. The router also has WISH, which is wireless Gamefuel, but that is for traffic between computers on your LAN. Like if you have a wireless media server, and want to give priority to certain devices to access it. Since everything is on your LAN, it isn't going from LAN to WAN, so Gamefuel doesn't apply.
5. Qwest is Cable broadband right? Then yes, you will still need your modem. But do you have a modem, or a modem router combo? What is the model number?