The base model comes with a 1.33GHz Intel Atom Z520 CPU, 1GB of RAM, a 40GB HDD, and a 3 cell, 24Whr battery. For $599, you get a faster 1.6GHz Intel Atom Z530 CPU, a 60GB HDD, and a 6 cell, 48Whr battery. The $649 version has an 80GB hard drive and a 2 year warranty (the cheaper models come with a 1 year warranty).
All three models have 12.1 inch displays, which makes the Mini 12 small for a laptop, but a bit large for a netbook. They also all run Windows Vista Home Basic, although they’re certainly capable of running Windows XP, or pretty much any flavor of Linux that you’d care to install.