The Wii is a B/G only device - It CAN'T do 5ghz.... Must to 2.4...
That being said, you can run a mixed mode, but your N performance will suffer... I run the router I replaced (wrt54g) as a G access point with a diffrent SSID, that way I can have a 2.4ghz G connection, and an N only 5ghz connection.
Also - Not all N adapters are Dual-Band... Make sure when using 5ghz that your adapter supports it!