When I first setup my DD-WRT router I had problems with Xbox LIVE reporting strict NAT and then moderate NAT.
To achive "open" NAT status on Xbox LIVE your best bet is ALWAYS UPnP.
Not even setting up manual LIVE port forwarding.
Both of these methods fall apart if you have two Xboxs. UPnP allows the Xbox(s) to decide the best ports to use, when they should be open and when to close them.
By default the DD-WRT v24 firmware disables UPnP.
To Enable UPnP on DD-WRT v24:
NAT / QoS -> UPnP ->
UPnP Service: Enabled
Click Apply. You may need to reboot your xbox for changes to take effect.
In order for UPnP to work as intended the client device (the xbox) must be using DHCP to obtain an IP address from the UPnP enabled router. To verify your Xbox is using DHCP, as opposed to assigning itself a static ip - open the dashboard and navigate to:
System -> Network Settings -> Edit Settings ->
Select "IP Settings"
To see what your Xbox thinks about your NAT try the "Live Connection Test"
System -> Network Settings -> "Test Xbox LIVE Connection"