There are two versions up for grabs, the Digital Standard Edition for $40 and the Digital Deluxe Edition for $60. The $20 premium gets you a "wealth of in-game goodies," such as a collection of exclusive helm - and weapon-transmogrification recipes to help distinguish your hero, a loyal Spectral Hound minion, three additional character slots, an in-game Treasure Goblin companion pet for World of Warcraft, and an assortment of Crusader-themed Battle.net portraits and Malthael-themed decals for StarCraft II.
Most of the Digital Deluxe Edition extras will be added to your account on the expansion's day of release, though Blizzard says some may arrive early, so keep your eyes peeled if you place a pre-order.
If you're looking for even more swag, there will be a Collector's Edition available at select retail locations for $80. It's not clear when these will be available to pre-order, but Blizzard warns there's only a limited number available and expects them to sell out quick.
The Collector's Edition comes with all the extras included in the Digital Deluxe version, plus a Malthael mouse pad, hardback Collector's Edition art book, and behind-the-scenes Blu-ray/DVD combo set with in-depth interviews with the development team and members of the Diablo III community.
You can pre-order either of the digital editions at the Battle.net shop.