Well, I'm lost . . . I have read this post several times, and I'm still not shure what you're trying to do. So let's play what if . . .
I take it that you are trying to connect two networks, both of which have have some kind of high speed internet access. If this is the case, you don't need a dial-up connection. You will some kind of a vpn server at one end, and a vpn client at the other end.
There are several types of vpn servers. You could go for a shrink-wrapped off the shelf thingy, you could use a Windows server, a linux server, or any number of solutions. You will have to have something.
On the client side, things are somewhat nicer if you want to connect just one computer to the remote network. Recent versions of Windows include a decent vpn client that will do just fine. If you want the entire network connected to the remote network, you will need to use a Windows or linux server. W2K or XP Professional are supposed to be able to share the vpn on the client side, but I don't think all of the security issues are addressed easily.
As for the firewall, if it is configured properly, you will have to punch a couple of very tiny holes. Depending upon the firewall, you should be able to do so in a way that doesn't create a problem.
Now, you get some homework: What os('s) are running on both networks? How are they connected to the internet? What are you using for a firewall? Do you have a budget for this project? What do you need to share across the network? . . . Post back, and maybe someone can steer you in the right direction.
