Mac Xbox Nat Problem

Okay, so first of all here's all my equipment:

1 MacBook
1 Xbox 360
1 Netgear Router WGR614v7 (Hardware Version:V7 Firmware Version: V2.0.23_1.0.23)
1 Cable Modem
VirginMedia Broadband

I use my MacBook as WiFi adapter for xbox to connect to live. The problem is that I always have a moderate NAT when I would really prefer an open NAT.

What do people suggest? I have UPnP but it still doesn't seem to work. Some people have also said it's because double NAT but I don't get if it's my router and mac or router and modem.

Please help, especially if you have this router as well (should be quite a popular one)