I just want staff in the back to hear what a soft spoken person is saying.
In that case, that cries out for a quality large diaphragm condenser microphone.
AT2020 USB (above)
For $130 you wont find a micrphone with better sensitivity.
Such mics, on mid gain, can pick up you rubbing your fingers gently together from roughly 10 feet away. Very sensitive.
Ive tested a lot of mics and done a lot of (too much) microphone research recently and purchased a lot of mics.
Also read this thread:
http://recordinghacks.com/2011/12/22/best-200-podcasting-condenser-mics/
The truth is however, for $30 more, getting the AT2035 is a superior mic, HOWEVER its not USB, rather XLR and you would also need
to purchase an XLR to USB converter.
As such a simplex setup is the AT2020 USB which gets its phantom power thru USB.