Just wanted to add something, in case it hasn't been said already...
When mixing device types (i.e. B, G and N devices) it is best to have a separate network (with diff. SSID) for each device class. They can daisy chain nicely with the WAN port of one router going to the cable modem, another WAN port of a router going into a LAN port of the first one, etc. Assuming, of course, you want to get to the Internet... NAT will probably block off the devices (of different types) from sharing things on the same network, i.e. Bonjour, Plex Media Server, etc..
This is because if (for example) a Wireless G device is put on a Wireless N network, the speed of ALL devices will drop to the lowest common denominator, so you won't be able to get the full benefit out of your Wireless N router. The same goes for B devices on a G network.
This is probably (anyone with more specific info, be sure to chime in) the case with 802.11ac, so even if you get your shiny new airport extreme, it may not be time to retire the old hardware quite yet.