I have always been fond of Atari's Jaguar and even -for a while- believed a Jaguar CD would make an excellent addition to my collection. I never knew of the Atari Jaguar II (or is it III?) though; not until I run into this pretty amazing Atari Jaguar 2 Prototype Board auction. This, as you understand, is an incredibly rare thing and the seller happily ships worldwide.
As for the Jaguar 2 itself, the prototype named Midsummer, well, as you must have noticed, it never reached a finished state and obviously never even came close to being released. It would apparently have been a powerful beast indeed, possibly coming close to the Dreamcast in terms of raw processing power. You can find out more about it here and here.