I used to work for a moving company.
Cold drinks (water, Gatorade, cokes), coffee and donuts in the morning, and some sandwiches and cookies, etc at lunch time go a loooooooong way towards encouraging the moving crew as well as the tips. Since the food comes first and the tips may or may not come at all at the end, why not build some goodwill with them? Nothing fancy is required, really - you can even just get a loaf of bread and make your own sandwiches for them. Trust me, they will appreciate it.
The amount you tip is up to you. If it's a particularly difficult move (stairs, long carries, huge furniture, etc) you might want to consider a bit more, but $10-20 per person would be ok, with perhaps a bit more going to the "leader" of the crew. It seems they will be moving only the "big stuff" so it probably shouldn't take too terribly long. As always, tipping is a personal thing - if you feel you got good (or better) service, then it should be rewarded.
Good luck to them with their move!
Cheers,
Deanna