iogear makes a nice little unit that we use all the time.  It's $149 retail without Audio, or $179 with Audio.  The new model also has an HDMI connection.  It's range is 30' and works well.

BUT....Your issue (like Bruce mentioned).  Each person using the unit must install software on their computer.  If outside vendors are coming to do presentations, it won't work.

What we typically do is use a iogear wireless unit for inhouse personnel, and a hardired vga/hdmi into the conference room table for outside visitors.