The amount of bandwidth you will need depends entirely on what program you use for the conferencing.
About 8 years ago we purchased a number of Polycom units. Great video/audio for a reasonable bandwidth (256kbps) but VERY expensive.
For another site we have here, we use VSee which has quite good picture quality, decent audio with built-in chat. It can also share the desktop or any application you are running. For our use, it runs at about 75-150 kbps if we are only sending video (Logitech Pro 9000) and audio. If you share a powerpoint presentation the bandwidth goes up to around 200kbps depending on the content of the slide being showed.
I believe VSee it comes with 30 days of free use.
Having recommended VSee, I must say that there are a LOT of other options out there, and we’ve tested a number of them. We do prefer using VSee if we want to send video.
If we just want to share a desktop and have an audio conference, we use GoToMeeting.