Leave your External Router (The WRT-54) the way it is (DHCP enabled).
Set the External Interface of your Core to obtain it's IP Dynamically.
I'm assuming you need the wireless capability of the WRT-150n as that is the only reason I can see having it there. If that is the case then you need to disable DHCP at the least and if possible disable any routing capabilities at all. Basically just turn it into an access point. I would actually recommend eliminating that router and replacing it with a wireless access point. Your Core is the router, putting another router between the core and your MD's only complicates the setup, there is no benefit.
Set the Internal Interface of your core's IP address statically, It should default to 192.168.80.1. As long as that is not the subnet of your WRT-54 internal network then that will work fine.
The Core will handle DHCP for computers on the internal network and the WRT-54 will handle DHCP for the External network. You can NOT have computers connected to both networks simultaneously, they will not work.
Here is an example IP configuration for your network:
WAN address: 126.96.36.199 or whatever your ISP gives you.
LAN address: 192.168.1.254
External address: 192.168.1.xxx whatever the WRT-54 gives you. (DHCP)
Internal address: 192.168.80.1 (Default for LinuxMCE this is the interface the Core will broadcast DHCP)
Disable everything router and turn this thing into an access point. Definitely disable DHCP.
If disabling routing is not possible on this box you must replace it as DHCP can't travel accross a router and you will not be able to assign addresses to the MD's that are connected to this router.
Clients connected to the internal network will be assigned numbers on the 192.168.80.xxx subnet and clients on the external network will be assigned numbers on the 192.168.1.xxx subnet.
Hope this helped.
Let me know if you need more info.