Video Capturing Cards - What to Buy on the Cheap for HD Quality?
The cheapest boards i know: 


the elgato game capture HD.
the BMD Intensity series. 


In that price-category you will need one board per camera. 

You don't need additional HDMI- or FiWi-cards. These boards can capture via HDMI, so there are ports for that signal.

Edit:
Yes, this boards can capture the audiosignal too.