need advice on a firewall for a large network

Hi everyone, i'm doing a little research for my company, we're going to need a firewall soon and I'm looking for suggestions on inexpensive / free solutions for our network. I know considering the network this is going to be difficult to find, but I'm going to try anyway. Below is an image of more or less what the network will be and where the firewall will be. It will have a lot of connections and major traffic going through it. Any suggestions on a solution are greatly appreciated

I can't see the picture of your network.

For a great free firewall with good documentation look at smoothwall (SmoothWall.org)

For a good not so expensive firewall look at watchguard's Firebox line

What is your description of a large network.