I have heard that there is some software that will let one computer take priority over others, but it would not work for an XBox, only on the computers. Some routers will allow you to set the percentage of the bandwidth that a designated device can use, but yours doesn't have that feature.
The one thing you can do with no problem is see what the signal strength is at where the devices are. The easiest thing to use for something like that is inSSIDer 2
. Install that on one of the laptops and take it around to where each device is and see what the signal strength is. Look at the graph on the bottom. The higher the line graph (the lower the number) the better the signal. Also I know it is counter-intuitive but depending on your wireless cards, the wireless router, and the way your home is constructed, being to close to the router may result in a loss of signal quality / strength.
If this doesn't work, let me know.