Hi lario,
I can't answer all your questions, but I hate to see a question in here with no responses. I can at least help with a few:
2) 7.1 = more channels to better cover a larger room that most of us don't have when seating more people than we usually do. That's my humble opinion, anyway. Instead of front, middle, rears and sub (5.1), 7.1 brings you front, middle, sides, rears and sub.
3) DVD is a long way from becoming obsolete, and when the high definition blue DVDs come out, they'll most likely be backwards compatible with DVD anyway the way DVD is backwards compatible with CDs. There are some specialty audio-only formats that take advantage of the vast storage capacity of a DVD, but format wars are keeping numbers down and the future of such formats uncertain. No impact on your DVD collection.
4) Newer DVD players have more bells and whistles, which depending upon model, can play: MP3s, one of the audio-only high definition formats, and progressive scan. Progressive scan only works with the new high resolution digital TVs, a.k.a. HDTV. If you don't have a new digital TV, then your old DVD player is still as good as it gets.
Hope that helps!