Help, Issues Connecting two Wireless Routers

Question

Ahead of time I would like to thank everyone for their responses and help. I tried to be as clear as possible. I believe I got all the details right. Some things are repetitive and might be a little confusing,  but please ask questions for any clarification. Thanks  for the help. 
Current Routers:
1. Linksys WRT600N ---- Used as primary router. Connected to the 4 ports are 3 PCs and a wireless router. All three PC have static IPs I reservered, also have a static IP reserved for the WGT624 wireless router. DMZ is also enabled and destination is the IP reserved for WGT624. 
2. Netgear WGT624 ----  WGT624 is connected to WRT600N through  the WAN port on the WGT624 to port number X on the WRT600N . I was using Netgear WGT624 as an access point for a Xbox360 , a couple of macs and  ubuntu netbook.  This router recently died needs to be replaced.
3. Linksys E3000 ---- Recently purchased to replace the Netgear WGT624 that has died. I want to use this as my new access point. Having trouble setting up the settings for this router. I want the same setup as before. 
Detailed Problem
I currently have the WRT600N located in the basement connected to a cable modem through the WAN port. Connected to the 4 ports on the back of WRT600N are 3 PCs and a Wireless router. Most of the settings on the WRT600N I left alone. The only thing I changed was reserving 4 IPs for the 3 PCs and the WGT624 wireless router. I also enabled the DMZ for the destination IP of the WGT624.  I also opened a couple of ports.
The Netgear WGT624 that died was located on the upper level third floor. I was using it primarily as an access point. It was connected through the WAN port  of the WGT624 to port X on the WRT600N which is located in the basement. Settings on this are a little different. On the WGT624 I disabled the firewall. I left both the Auto DHCP enabled and the DHCP server. WPA was enabled on the WGT624. With these settings on the WGT624 I was able to connect to a xbox 360 and a few macs wirelessly to the internet with no issues .  I also noticed that the the internet IP displayed on the status tab on the WGT624 was the IP I had reserved on the WRT600N.  
Recently the WGT624 died after  years of usage. So I purchased a Linkys E3000. I want to have a similar setup as before with the WGT624. However I have been running into issues. Wirelessly I cannot get access to the internet through the E3000. The E3000 is connected to the WRT600N in the same was as the WGT624. The WAN port of the E3000 is connected to the port number X of WRT600N.  On the status tab of the E3000 I seem to be getting a strange Internet IP address. 127.0.0.1. This I think is a loopback address, I am not sure? Same with the subnet mask of 255.0.0.0
So the problem is I want the keep the same setup as before if I can. I don't want to the any computers who connect to the access point to have access to any computers that are connected to the 600N.
I read on another thread I could connect two wireless routers together by connecting port X on the 600N to port X on the E3000.   Then changing the internal IP of the second router from 192.168.1.1 to 198.168.1.2 and then disabling the DHCP server.
I really want to keep the same set up as before if posslble. More importantly I want to know why my setup prior, the WRT600N  to the WGT624 worked with no problems. Why could I use the WGT624 as an access point and not the E3000? Should I switch the the two routers use the 600N as the access point and E3000 as my main router?
Thanks again.

Answer 1

1. You have chained two routers. The WRT600N uses LAN IP address 192.168.1.1 subnet mask 255.255.255.0. The Netgear is connected with the WAN port to the WRT (thus uses an 192.168.1.* address on the WAN port) and uses 192.168.0.1 by default on the LAN side. Your new E3000, however, uses the Linksys default 192.168.1.1 on the LAN side. If you connect the E3000 to the WRT600 without making any changes it won't work. A router cannot work if WAN and LAN side both use 192.168.1.0/255.255.255.0. That's impossible.
2. Thus, the quick and easy setup to get the same as before would be to change the LAN IP address of the E3000 from 192.168.1.1 to 192.168.0.1. After that change you can connect the internet port of the E3000 to the WRT and everything works like before.
3. However, in particular because you need port forwarding and you do port forwarding through two routers I would recommend not to run two routers chained. I can't see why it is necessary for you to run two routers and two separated networks. The basic setup and an explaination why a different setup may be better is here. I think in your case it would make things much simpler and you have full connectivity between all your computers. You can set up a roaming wireless network, too. The only problem is that you will "loose" one port on the E3000 because you cannot use the internet port.