How to check how much bandwidth users use in a network?

This is my setup: I have verizon dsl with 7.0 mbps of download speed, and 0.750 mbps of upload speed. I use a westell model thats connected to a wrt54g2 linksys router. The port 1 of my router goes to my computer, then the second and third ports go to a ps3 and a wii. I also have a person in another room getting the wifi signal from my router in his computer. I want to know if its possible to check how much bandwidth the user with the wireless pc uses. I think he is affecting my connection speed and input lag when I play with my videogame consoles online. I went to another site and they recommended me to use the QoS settings in my router. I did that and inserted the mac address of the wireless computer into it, and set the connection type to “low”, while I set the one for my ps3 to “high” ( and also the wii through its ip address to “high” ). The thing is that I still dont know how “low” bandwidth usage that user can get, so some percentage or numbers will really help here. Some people also told me that even though setting a network with QoS wont exactly prioritize internet traffic, but just fix it at certain values. For example, I want the wireless computer user to get a “low” ( slow? ) connection ONLY when I connect online with my videogame consoles. When I dont have the latter connected he should use the bandwidth in an optimal way. How do I set this up, where in windows or in my router can I check this data, or what program do I need to download in order to check bandwidth usage?
Thanks in advance.