hrm...i'd stick with the edition listed, sadly enough. with fields like soc and psych and anth, especially, you're going to have lots of new field data and case studies in the newer editions - especially when the publishing dates are five and ten years apart. i know it's painful, but check out amazon and ebay and half.com and you SHOULD be able to find the books for at least slightly less than the cover price.
as for waiting, there are pros and cons to that strategy. pros - sometimes professors change what books are needed, and you don't want to end up buying more than you need. this hasn't ever happened to me, but i know people it has happened to.
cons - if you wait TOO long, the cheap books will have all been scarfed up and you'll end up paying full price anyway. or they won't arrive until a week or two into classes, and in many classes, you'll need all your books from the first day onward. i prefer to start early and grab all of my books for ridiculously low prices. and remember - when you're done with them, you can always sell them [but NEVER to the bookstore unless you want to be royally ripped off]. also, rest assured knowing that, unless you're going to be a science major, prices for books generally go down the higher level classes you take. intro level classes want you to have textbooks that cost a fortune - upper level classes generally require one or two $15 novel-length books. as with many things, it gets easier as you go along