I have discovered an issue with Windows 7 64 bit OS. My attempted set up was to havea 802.11n Belkin Play router connected to my Actiontec MI424WR router as a wireless access point with the MI424WR wireless radio turned off. This worked fine for my XP 32 bit PC, my Windows 7 32 bit PC, and my 2 Windows 7 64 bit laptops using wireless. A problem arose when I tried wired connections on the Windows 7 64 bit laptops and a Windows 7 64 bit PC.
While wired, internet would drop either completely or intermittently but too frequently to be useful. Tech support had no idea what to do or what caused it. It was reproducible after a reset of the router and then turning off the wireless radio.
I could fix it by turning on the wireless radio again, but I had hoped to turn it off and only use wireless from the 802.11n router.
Did you disconnect wireless on the 2 Windows 7 64 bit machines while testing wired?
What is the connection between the Actiontec and the Belkin? LAN-to-WAN or LAN-to-LAN?
If the connection is LAN-to_LAN, the wireless connection on each machine would have obtained a DHCP address from the Actiontec. When you connected wired, you would have gotten a new IP address based on the different MAC address of the wired NIC, resulting in the machine having 2 IP addresses. This may not be readily apparent on the Actiontec main status page as the second connection will get a generated name of "new-host-xx" (the router can't reuse the same PC name for 2 different IP addresses). Windows should be able to keep the two connections on the same subnet straight, but I've seen WIndows do stranger things.
If the connection is LAN-to-WAN, the Belkin must have a different subnet address than the Actiontec (e.g.. 192.168.2.x). Now, if you connect both wired to the Actiontec (192.168.1.x) and wireless via the Belkin (192.168.2.x), Windows is going to have connections to two different subnets with two different gateway routers. Without explicit routing rules, Windows won't know which gateway to use.