the ports to forward are
UDP 88
UDP 3074
TCP 3074
You may also want to make sure you have the latest firmware for your router.
Also, my downloads usually start on 5% then after a couple minutes start going up. It depends on your connection speed and the size of the download though.
It is also important to either forward this ports on both your router and your modem, or place your router in your modem's dmz
With this setup, I have lag free games, fast downloading on marketplace, and can talk on live
Here is the link to the ports on the xbox.com support site
http://support.MS.com/kb/908874/en-us Also, if your search xbox-scene for "xbox live ports" the first link brings you to a discussion just like this one, where I gave the same information.