1) Yes, although you could use the Display Ports
2) They are a relatively new type of connection, high bandwidth and can carry things like USB as well. They are the open standard equivalent to HDMI (which is licensed).
3) That is because they are 16:9 aspect ratio, while yours is 16:10. They are also cheaper panels for the most part, you can still find higher resolution versions (Ultrasharp U2411, HP Z2040W).
4) Depends, I believe that you can do either, although if you use a Display Port to DVI adapter, and you connect more than two monitors, it will need to be an "active" adapter.