Itinerary suggestions

You've caught the bug, enjoy! I am seven trips in and would go back tomorrow.
Just some random points to consider: If you can fly in to Milan and out of Naples or Rome, then I would be in favor of including both those places, but I would just hate to hop on a plane in the middle of vacation. You will lose considerably time by moving that far. I strongly favor covering one area well over doing north and south in one trip, but that is just a personal predilection. I also rarely see the point of one night stays--people that rush through Venice are often the ones that say they hated it. I might save that for your third trip... People usually say it is better to not end in Venice because the outbound flights are so early and it's a pain to get to airport, so I would take that into consideration.
Four nights would be the absolute minimum for Naples and the Amalfi Coast as it takes a while to get there and also to move around.
One thing I always do while planning an itinerary is look at the travel times. I am not going to spend all day on a train when I could be somewhere else that is equally awesome in an hour. Google Maps can give you quick shortcut views, but also look at Trenitalia (probably Florence or where ever you drop your car from Arezzo to Naples or Salerno (in Salerno you can access ferries to Amalfi--so it depends on whether you do Naples or the coast first or second).
Just some things to mull over! Is Arezza an absolute lock at seven days?

Anita: The cheapest price on airfare is not always the best value. Many travelers will fly open jaw into one city and out of another to (1) save Time and (2) keep from backtracking which can be very expensive. We seldom fly into and out of the same airport.
You might want to go onto Google Maps and see how far it is from Milan to the Amalfi Coast, up to Venice and back over to Milan. That's more miles on a train that I'd want to spend. Distances in Italy are much longer than many people realize. If Arrezzo is a priority for you, Rome would probably be the best airport to fly into.

Thank You, for all of your comments and suggestions. It is nice to have them as they will always help out. And got the bug! You have no idea, my family was ready to pack up and head there for good! it is an amazing place.

Yes, it can be difficult to tear yourself away from Italy. There is so much variety there.

I would discourage a super-short visit to Venice. That more or less means you'll spend all you time at a few key tourist spots, surrounded by thousands of other visitors. Many people who do that end up not liking Venice. The way to enjoy the city is to give yourself some extra time so you can wander around the little canals, away from the hordes.

However much time you are planning in Naples, double it. I am in Naples now and the city, while crowded with tourists, is buzzing with life. It is the thrilling and complex international Mecca it is reputed to be. I am on the terrace of my hotel, munching on taralli while I sip my Cuban rum and smell the sweet flowers which are, incongruously, still in bloom. The food is great, the views spectacular, and the cabbies will scare you beyond belief. Do you like New Orleans? This is better.

You are definitely trying to cover too much ground in too little time. When you compare airfares take into account the cost of train tickets to backtrack and the extra time.

7 nights is only a full week if you don't change locations. To go from Arezzo to Naples/AC to Venice to Milan uses the better part of 3 days in travel. Arezzo-Naples minimum 4 hours on the train, Naples to Venice 5 hours . . .