Airport Express Manual Setup

I'm running an Airport Express in my wired dorm room. To work properly here with the school network, the router can not assign multiple IP addresses. Currently the router is on Shared IP. I couldn't get Bridge Mode to work. What I'm having issues with is when multiple WiFi capable devices are connected only one (usually my MacBook Pro) can actually get through to the network at a time. I originally brought my Airport Express for two reasons, AirTunes (works great) and so I wouldn't have to constantly have to plug and unplug a Cat5 cable between my computer and Xbox. Anyone have any suggestions on how I can set the router while keeping the ability to keep my computer connected to the internet while wirelessly accessing LIVE?