Help with adding another router and network to single connection

I am connected to the internet through U-Verse 2Wire residential gateway and have this connection DMZ to a Linksys WRT600N. From the Linksys one port goes to a switch downstairs that connects an X-Box, Blu-ray player, and media PC, another port goes to a Micro-Cell 3G, and another to a Windows 7 PC. I also have 3 cameras and a couple of laptops connected wirelessly to the network.
I just finished building a test box for Windows SBS 2011 and want to set it up on a separate network. I have a WRT54G v4 that I have just flashed with dd-wrt v24. I am hoping that I can connect it to the remaining open LAN port on the wrt600n and create a separate network for SBS 2011. I would then want to connect a couple of wired and a couple of wireless computers to the SBS 2011 network for testing. I would also like to have internet access. It isn’t necessary to see or access the other network.
I have been able to setup what I have so far from searching the internet, but I am having difficulty finding the right directions to go farther with adding this additional router and network. I'm really confused on how the subnets, DHCP, etc... needs to be setup. Any guidance will be greatly appreciated!