The Kindle 2 includes built-in 3G wireless, which lets you download titles from Amazon in about a minute flat. If you can't tap into the 3G network, this model toggles down to Sprint's slower 1XRTT data network (if it's available).
Amazon has ramped up the on-board memory to 2GB, so the new Kindle can store up to 1,500 books at a time. If you run out of space, you can delete purchased titles from the device's memory and then re-download them later at no additional charge. However, there's no expansion slot for additional memory.