I recently replaced my Cisco 877W with a Fritz!Box 7390, because I'm a tart, and I wanted built in gigabit ethernet, 802.11n, IPv6, and a whole bunch of other fun stuff.
It was a bit of a fiddle to get it connected up to my O2 Broadband connection (as O2 uses totally different ADSL settings to most UK ISPs), so I figured that I should probably document the connection settings somewhere, ready for the day that I tweak something a little too far and have to factory reset everything ;-)
(These settings are all with expert mode turned on, so remember to turn on expert mode before whinging that you can't find one of these options).
On the Fritz!Box 7390, click "Internet", then "Account Information". Settings as follows:
Connect Via: "Internet connection via DSL"
Operating Mode: "Use one Internet connection for all computers (router)
Account Information: "No account information required (in accordance with RFC 1483 / RFC 2684)"
Encapsulation: Bridged (also tick "Obtain the IP adress automatically (DHCP)")
Select "Set the speeds of the DSL line automatically".
Things to note, these settings work for me, YMMV ;-) These settings are for the O2 Home Broadband LLU product without a static IP. Obviously, if you have a static IP, or use the non-LLU O2 connection, these settings will require modification.
[UPDATE: I added my IPv6 stuff on a new post, Fritz!Box 7390 Hurricane Electric TunnelBroker.net IPv6 tunnel, enjoy]