As far as I understood Apple's documentation, the extension feature only works with the more recent AirPort products -- Extreme and Express. Indeed, the ability to add multiple base stations to a network without needing to wire them physically to the Ethernet network was added with the release of AirPort extreme.

I guess that AirPort Express would extend the range of an AirPort (snow or graphite) network, provided that it too is wired to the network...

F.J.