I have to contradict the 2820n not being able to have to seperate SSID's running at different speeds. I agree you can't state you want one speed on one and another speed on another, but if on one SSID you enable WPA2/PSK that effectively enables it for Wireless N and if on the other SSID you pick WPA then you are effectively enabling that for only 54MBPS.

That is how i have it set up at the moment.