From what I understand, if you normally fly heavily discounted fares and want to upgrade, DL is not the place for you. Go search the DL forum for the phrase LUser for tons of details and folks that can explain it better.

If you avoid F-100s, you should do pretty well for upgrades on American. Just as importantly, with their MRTC program you'll be reasonably comfortable in the back as well. You should be able to upgrade a good percentage of the time on BWI-ORD or BWI-STL, but it'll be a bit tougher on transcon flights.

Also, don't forget Midwest Express if you value upgrades and you can get good flight times from DCA. While their route network is small, my friends that have flown it rave about their in-flight service.