MS ISA Svr

You need to just create a new "Website Publishing Rule". If it is port 80, and that is not already opened up you also will need to open port 80 on the external Cisco firewall, and create a new ISA "Web Listener" for Port 80 and no authentication. But the second part can be done within the actual publishing rule creation steps.
In ISA, expand server, right-click Firewall Policy, select New > Web Site Publishing Rule
Give it any name, choose allow, leave the top option and click Next again
Configure to use "non-secured connections", assuming this is a public HTTP site, and click Next
enter the internal site name, such as the www.publicname.com (not really the internal site name); then also check the box and enter the internal computer name/IP so it knows where to send the traffic, and click Next again
For path enter "/*", check the box for forwarding host header, and click Next
Enter the public name again, click Next
For the Web listener, click New, dont' require SSL, next again
Use External, and click Next, use No Authentication, click next again, and then next through rest of wizard
Try it out, but also refer to http://technet.microsoft.com/en-us/library/cc767295.aspx
 for more information
 

where you say to put "www.publicname.com", is that just what I want to publish it as?  

I don't actually have a public dns name for the site yet, so I was going to just test-navigate to it by typing in "http://x.x.x.x" (public IP assigned to it)

That being said, should I type in x.x.x.x (public IP) for the internal site name?