One of the most interesting features of the Linksys E3000 is the fact that it can provide simultaneous dual-band networks in your home.
An old wireless-G router would usually provide a single network at 2.4 GHz - with one SSID and one passphrase to connect. Everything on the network would then use this one connection.
These dual-band routers allow you to create two wireless networks - with a separate SSID and passphrase for each. One will operate at 2.4 GHz, and the other at 5 GHz.
If you are comfortable with the advanced setup page of a router, then the manual setup of the dual networks is fairly straightforward. The setup page just has the option to create each one individually. Just make sure you give each a useful SSID so you know which is which.
There are a number of reasons why you may want to do this - and the main reason suggested by Linksys is to keep different types of traffic on separate networks so that they don't interfere with each other.
For example, you could setup your faster 5 GHz network and use this just for devices that stream video and audio files around your network - or keep it for other high priority traffic like Skype. You can then make sure that the 2.4 GHz network is used for all other traffic on the network like general internet use and your guest access.
Only wireless-N and wireless-A devices are able to connect at 5 GHz - but be careful because cheaper wireless-N wi-fi cards may only support the 2.4 GHz band (like the one in my computer does!).
For testing the 5 GHz band in this review, I used the new Linksys WUSB600N dual-band wireless-N USB adapter, which can be connected to any computer so that it can connect to 2.4 or 5 GHz wireless-N networks.