IME, it's always been very bad to have more than one NIC connecting to the same network, (eg. wired and wireless, 2 x wired, etc). The system ends up getting confused about where to send things.
Probably requires some fiddly settings to get the multiple connections to load share with each other or you can use something like Speedify or Dispatch.
-4wd
Using multiple combined NICs is usually referred to as "teaming." (In the NIX world it's also called interface "bonding.}
NIC teaming is not a feature built into Windows 7. There's an "intermediate" driver that needs to be installed to allow teaming. So getting it to work depends on the brand of NIC (and driver) installed - plus a network switch that supports NIC teaming. I know Intel supports teaming on some of their NICs. There are likely other brands that also support it. But I'm not familar with them. (I've been told Broadcom has teaming support for some of its adapters.)
FWIW I've never seen NIC teaming done on the PC level. From my experience it's purely a "server thing." 
So...is there anybody out there in DocoLand teaming NICs on their PC or laptop? 
-40hz
Note: Windows 7 DOES include the ability to set adapter priority for multiple interfaces.
Using this capability you can set it to use a cabled in connection in preference to the wifi, which makes it so that you can actually be connected to the wifi and the cable simultaneously without issue.I have my laptop set this way, and can plug into the cable for more speed without having to turn the wifi off. The default priority in windows is backwards of course, preferring the wifi over the cable for some strange reason.
What you don't want to do is bridge the nicks in windows when you have more than one connected to the same network, as this results in a layer 2 loop that can and will bring down the whole network.
As for wifi not reaching full speed, a lot of times this has to do with signal to noise ratio. Even though it is getting a strong enough signal, there's too much interference for it to really be heard. OpenWRT has the capability to display the SNR on a per device basis, which is really convenient for troubleshooting situations like this. It also shows transmit and recieve data rates, again on a per device basis.
One last thing to check- you are using WPA2 encryption correct? Wireless N will not exceed wireless G speeds unless it is encrypted using WPA2 or better.
As for IPv6, most of the time it does not matter. However in some cases your system will get a valid IPv6 address that does not have a usable route to the rest of the world. If this happens it will try to use that address since Windows prefers IPv6 over IPv4 by default, and the result is a huge slowdown in browsing performance because it has to wait for the IPv6 attempt to time out before falling back to IPv4.
You could just disable IPv6 until your ISP implements it the rest of the way, or alternately you could change the IP priority so that Windows will use IPv4 in preference to IPv6- eliminating the risk of slowdown without removing your ability to experiment with ipv6.
