Never used VNC, but given the situation:
Foward the ports that VNC uses to the local IP that you need it to go (ipconfig to find out the ip... you know the deal).
To access it, i'd bet you'd use
http://whatismyip.com/ to find out your ISP given IP (external), then 205.127.11.173:555 (the first part before the colon being your IP, then the port number after the colon) and it should bring you to it...
then again, i've never used VNC.
Edit: to use multiple PC's, there should be a option in VNC to make it use a different port? If so, you'll have to configure each installation on each computer to use a differnt port
ex
PC1 - 5675
PC2 - 8483
Pc3 - 8888
PC4 - 64363
Then in the router settings foward each of those ports to the coresponding PC's...
But like I said, i'm no VNC expert.
42 - The meaning of life.