I have the airport express as router can I buy the new airport express and use my old airport to extend my net?

Yes. Please check out the following Apple Support article for the options available for you to extend your AirPort with additional AirPort base stations.

Yes. How you go about doing so will depend on the model of your current AirPort Express. If it is an 802.11g version, you would have to configure it for a WDS. On the other hand if it is an 802.11n model, then you can use the extend configuration.

I have an old AirPort Express model in addition to AirPort Extreme. As far as I know, it cannot be used as a base station to extend my net. The only way to extend my net is to buy an AirPort Extreme. But from what I have read from the official Apple site, the new AirPort Express not only has simultaneous dual-band 802.11n support, but it also seems to function just as the original Airport Extreme, only with less LAN ports.

"If you already have a wireless network in your home and want to extend its range, AirPort Express can help. Just place it in range of your primary base station — an AirPort Extreme, Time Capsule, or another AirPort Express — and near the area where you want your wireless connection"

Things can get confusing pretty quickly since there are 3 different versions of the AirPort Express.

If by "old", you mean you have Model No A1084 or A1088 AirPort Express (look at the label on the side of the device for the Model No), it cannot be used with the "Extend a wireless network" feature of other newer Apple routers.

If your AirPort Extreme is "square" shaped with rounded corners, then the "old" AirPort Express will not work with your AirPort Extreme using the "Extend a wireless network" settings.

You could use a "previous version" AirPort Express, Model No A1264 to "Extend", or the "new" AirPort Express, Model No A1392, for that purpose.

If your AirPort Extreme is a simultaneous dual band product, then the "new" Express would be able to "extend" both bands. The "previous" AirPort Express (Model A1264) would be able to extend only one band, since it is not a dual band device.

I have the "previous" AirPort Express (Model A1264) , which is not a dual band device. I hope Apple could issue a firmware update to upgrade A1264 to a dual band device. I wonder if it is technologically feasible.

No possible firmware can be able to do this, as it's physicaly impossible to alter the hardware this way. If it were possible, there would be no need to purchase or design the 2n'd gen in the first place.

Thank you. So the only advantage for the "previous" AirPort Express (Model A1264) is its look when plugged into a wall socket directly. The latest model would look ugly while dangling from an electrical outlet in the living room. Just look at the way it is designed.