I originally bought two cards at once but upgraded just the one in the controller ('cause it is the easiest of the two to replace) and did not see much if any improvement. I then did the one in the Solo and and got a huge improvement. I did a flight as far out as I was comfortable given the lighting conditions over a frozen lake and exceeded 3200 feet with two bars left (on stock antennas).
I ordered from
here, took just a couple of days to arrive. Also if you've not already seen it
this thread may be of use to you.