OK I know this post it old, but just in case you didnt figure it out or for anyone else that could use this info including me if I ever forget. Here it is.
I had the same setup. I'm going to assume you are running custom firmware. What you need to do is
Router connected to modem = Gateway
Router connected to 360 = Bridge
--Bridge Settings--
-----------------------Setup Tab---------------
Internet Connection Type : Static IP
Internet IP : 192.168.1.X(I have mine set to 87, i dont know y i just do)
Subnet Mask: 255.255.255.0
Gateway: the IP address of the Gateway
Static DNS1: Enter the DNS1 found on a computer connected to the Gateway
Static DNS2: The IP address of the Gateway
-Optional Settings(<not optional)
Host name: Enter the SSID of the Gateway
The rest leave default
-Network setup
Ip address: whatever you want THIS routers IP to be(I like to set mine up high to avoid any possible future IP conflicts)
Subnet Mask: 255.255.255.0
DHCP Server: DISABLE
-----------------------------------------Wireless Network Tab----------------
Wireless Mode: Wireless Ethernet Bridge
Wirless Network Mode: Mixed
Wireless Network Name (SSID): The one from the Gateway
((Make sure if you have any wireless security enabled that they match up))
-------------------------------------------Security Tab---------------------------
Firewall Protection: Disabled
Leave the rest default
That should be it...If you have any problems or Q post it...and I will see what I can do.