The only other setting I can think of is the Flow Control, which is also located in the Advanced tab. This is another one of those, "It should work......there appears to be no reason why it won't". If you do install a NIC card, let us know how it turns out.
Have you checked to determine if Realtek has an updated driver for your controller:
http://www.realtek.com.tw/products/productsView.aspx?Langid=1&PNid=13&PFid=5&Level=5&Conn=4
Have a look at Control Panel ....Device Manager to see if the controller is listed as something like 8167/8111X controller, where the X is probably a version number. That might match up with the second link. I can't find an controller with "8167" on the Realtek site.
Have a read through this link:
http://www.sevenforums.com/network-sharing/42614-degrading-internet.html
Basically it alludes to Realtek controller problems possibly caused by IPV6. If you go into your Network and Sharing Center, and select Local Area Connection to bring up the status panel you will be able to see if you have an IPV6 address on your pc. When you are connected to the CGN3 you probably will not see an IPV6 address. If you connect to the Asus router, you should see that IPV6 address given that the RT-N66U spec indicates support of IPV6. If you uninstalled IPV6, you might see if in fact there is any truth to the suggestion that this is causing problems. The properties page is located on page 14 of the RT-N66U manual. An alternate idea, and probably easier one is to turn off IPV6 on the N66U, and then reboot both the router and pc.