It's can't do N. It is possible that an N card will be released which will fit the new slot as it is more or less a standard (in terms of connection and size).
One big problem is that the new card isn't as easy to get to as the older ones were. This means that to replace the card will pretty much require a trip to your local Apple Authorized Service Center.
The old Airport cards (both original and Extreme) were Broadcom chip based and are b and g (Extreme only) compatible. The Intel Mac Airport cards are Atheros chipped and are a, b, and g compatible.