Video Capturing Cards - What to Buy on the Cheap for HD Quality? Im new to the video and capturing world.  I want to give you the setup.

We will be filming a event with three or two cameras. Then pushing this video to a webserver and stream it live.  We will be using Silverlight HD technology which can stream out at 720HD quality video.  

Most likely two cameras will be over cable and then one over wireless.  We currently have not figured out what video cameras we want, but to save on costs, we plan on renting the equipment.  Our budget is also $1,000.  We can push it over the top a little, because once we have the hardware, we will be using it over and over hopefully recouping the costs.

We have a PC figured out for encoding and that will be a desktop with a GTX NVidia Card for GPU processing.  The CPU will run us $550, so over and done with.

So we have about $500 or less for the capturing card(s).


First Question, Can we get away with one capturing card for three cameras?  Or do we need one card per camera?
Second, What cards can we buy on the cheap?  If we need three cards, What cards can we buy for about $125 a piece that can produce HD quality?  If we only need one card, Whats a good card to buy that can produce three video feeds?


All the cards need to connect to a Desktop Machine.  It doesn't really matter how, as we can buy Firewire cards or HDMI cards if needed.

EDIT


Do we need to capture Audio Too? Or will the Capture cards do just that?


Thank you!
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.