Dual Gigabit Ports - Useful?

That depends on what you mean by connection redundancy. What is your end goal? If one goes down you want to use the other, or you want to use both simultaneously? Are you running a server?

I'm not exactly sure, since I've never directly tried to do what you want. The only issue I'd wonder about is whether Windows will be smart enough to consider a connection down if the ethernet connection is still up. In other words, if the DSL connection is unplugged from the phone line but the modem is still plugged in and Windows still sees an ethernet link, I don't think Windows will consider that connection down, so it would probably still try and use it. If the modem physically got fried by lightning and the ethernet link to the computer was down, then yes, windows would use the other connection exclusively with no problem.