What's not having the 802.11n standard do?
Alfa does make a b/g/n adapter and I can agree with skaz that it's one of, if not the best range extension usb adapter in the world. The only difficulty I've found with it is that sometimes the drivers aren't regularly updated so that the user interface leaves a little to be desired.
Anyway you can find a link to the Alfa's drivers page on this review of the Alfa adapter
Skaz was talking about. You'll be able to see if there's a 64-bit driver for any of them.