The question is how do you want to cable the netgear and what do you want to do with it.
If you cable a lan port of the DLink to the Netgear WAN port you will have two subnets that must be addressed differently. Opening up and forwarding the ports for XBox could be difficult.
I believe cabling the Netgear as an access point makes more sense. CONFIGURE THE NETGEAR BEFORE CABLING. This is achieved by wiring a lan port on the DLINK to a lan port on the netgear and ignoring the wan port on the netgear. This will involve configuring several router items that should be completed before the devices are cabled together. You will need to remember the Netgear's new network address and probably use a static IP address from a computer to configure it back into being a router after these changes, if required. Or reset the configuration to factory default following the users guide.
First the Netgear needs DHCP turned off as it will be part of the DLink's network and not a separate subnet - the DLink will perform the DHCP services.
Second the LAN side of the Netgear will need a static IP address in the DLink's network. This is where it may get tricky for you. Verify the DLink has an IP Address like 192.168.0.1 and a subnet mask like 255.255.255.0. That means all IP Addresses in your network start 192.168.0.??? where ??? is unique for your network. Verify your DLink router has a set of addresses reserved in the DHCP setup for dynamic address assignment these addresses like 192.168.0.100 to 192.168.0.199. If the above is true or with the information you extract from you DLink router you need to determine where you can create static IP addresses -- in the above case 192.168.0.2 to 192.168.0.99 are available. You need to assign one of those like 192.168.0.2 to the LAN side of the NetGear Router. You will probably want to assign another to the XBox == like 192.168.0.20. You will probably need to deal with opening ports or placing the XBox into the DMZ of the DLINK router which will require a static IP Address.
Now cable the devices
When considering the XBox Configuration below the DLink is the router that must be configured to allow the open ports or DMZ access to the Xbox == the netgear is not functioning as a router in this configuration.
Check out the following links on XBox Configuration if required.
The second link may be the best to start with.
The generic connection and configuration documenthttp://www.microsoft.com/windowsxp/using/networking/setup/xbox.mspx
This is a good description of possible router
If you need assistance setting this up in your environment leave a comment on this problem and I will respond.