I'll second NetFixer
's advice... you are usually better off with a simple modem (the SB6141) and a separate router that you own if you need to hook up multiple devices.
There are multiple issues with cable modem gateway devices (built in router/wireless):
Your ISP controls your firmware. If your ISP refuses to push an update to you, and you have problems with any functionality, you are out of luck. The same applies to a basic modem, but there's less to go wrong with those. With your own separate router, you can update the firmware to add functionality or fix bugs (or just replace it if needed).
You can get more features with your own router if that matters to you. Most cable modem gateways include only basic features.
Your ISP can lock you out of certain features - for example, they can disable the wireless if you don't pay for their wireless service (this is evil if you own the modem, but they can do it if they want to).
Cable modems can last for many years. Wireless standards change quicker. The wireless in the modem will probably become outdated before the modem is ready for replacement.
You can put the SBG6580 into bridge mode and use it as a basic cable modem if you ever need to, but honestly it is just easier to stick with a basic modem.