Doesn't the 360 have a browser?
From all machines when nothing else is happening on your network, test your Internet performance...
Find one that works on the X-box and then use it on the other machines.
http://performance.toast.net/
or
http://www.dslreports.com/stest
If the results are within 10-15% of each other, it isn't your wireless.
Once we get this resolved, if you don't want a 100' cable on your x-box, you can get a wireless adapter for the 360 or a stand alone wireless bridge may be an even better idea in case you get something else near your TV that needs a network connection.
I'd be happy to come check your network out if you want. It'll cost you dinner at a meet though...