First, check flight availability on your flights with this tool:
http://www.flyaow.com/classcwt.htm
If it shows "Y0", then NW is no longer selling tickets for this flight, at any price, to anyone (other than Plat Elites, who are guaranteed seats on sold out flights more than 48 hrs out.) If there is any number after the Y, then you are not yet overbooked, and chances are good that there are no bumps.
If it does show Y0, arrive at the airport 3 hours early, rather than 2, and tell the check in agent that you'd be willing to take a later flight if necessary. You want to arrive early so that you are on the top of that list. When you get to the podium, check again with the gate agent.
If you are bumped, you can expect to recieve either a $200 credit or a $300 credit; it depends on how long you have to wait for your next flight (I think the cutoff is 4 hours, but don't know for sure.) Alternatively, you may be offered a US48 round trip voucher. That voucher is heavily capacity-controlled, though, and can be tough to redeem, so you want the credit for a future flight instead. Also, ask if there is a way you can call ahead to let your family know you will be late. This should get you an "amenity pack", which contains a certificate for free freq flyer miles, a coupon for $X off a flight of $Y (for several different values of X, Y), and a 10 minute phone card. Finally, ask if you can be seated in First on your new flight, and try to get boarding passes printed right then.
If you have a later flight in the day, you need to know that it is possible you will have to stay overnight. In this case, NW will put you up at a local hotel. So, weigh this in your decision.
Finally, don't get your hopes up. NWA has been getting pretty good at predicting noshows. Most flights I've been on have had at most one or two empty seats, but I haven't seen a flight have to bump anyone for over a year, and I fly at least monthly.