Just for another train of thought. I didn't want to replace the internal card and deal with the interior of the iMac, so I got a Netgear R6250 router and placed it in bridge mode. I then connected the bridge to the iMac with an ethernet cord.
I found using the latest time capsule, that I had issues with a stable AC wireless bridge connection until I renamed the 5ghz AC band to a unique name (using Airport Utility.) After I did that, my connection has been rock solid.
Of course, I don't have all the other great features such as handoff I could have by opening up the iMac and using the above kit, but at the time the kit did not exist, and I still question my ability to open up the iMac without causing damage during the upgrade.