truth is streaming using uPnP or DLNA server (packet video) required very little bandwidth unless the video you are streaming is raw HD rip i.e. it will need 50mpbs or something even higher to stream. Most MKV (1080p) AVI and WMA HD videos requires only about 8-10mbps which is easily done over even a 801.11g wireless (54mbps) connection.
but then again I can't find any TV can play MKV files directly (with a reasonable price tag), which is stupid as MKV is the most common HD file container format.
To answer your question, 100mbps is more than enough (my NAS is connected to my router via 100mbps and it is fully capable of streaming to my computer, laptop, phone and tablet simulanously - yes i tried it) but the question is not how much bandwidth required, but "If you can do it". Sadly the only way i know without forking out thousand is to get a Networked Media Player or play it off a laptop (which is what i do and I like it cos the laptop act as a media player and puts HDMI signal to my AV receiver)