The Hard Tech Specs on the Atari Jaguar are quite impressive. The "CPU", if it could be called that, is a set of 5 'processors' on 3 chips. There is a standard-issue M68000 for doing the uninteresting system functions, with the other 4 processor cores living in pair of proprietary chips going by the monikers of 'Tom' and 'Jerry'.
Tom is a 208 pin package with 750,000 transistors. It's first core is the GPU, a 32 bit RISC core with 64 32 bit registers and the ability to read 64 bit chunks in one swipe from the 64 bit system bus. It's clock is measured at a hair over 26mHz.
The second Tom core is the Object Processor, a 64 bit core (also RISC) with more 64 bit registers and hardware support for sprite manipulation and character mapping and pixel/voxel management.
Number three in Tom is the 'Blitter', which is yet again a 64-bit RISC CPU for logic and maths operations like shading. #3 also serves as the memory controller.
Jerry is an altogether smaller design at 144 pins and 600k transistors, and encompasses the DSP. Though primarily for sound generation, Jerry uses the same RISC core as Tom's GPU and so with a wizard at the helm, Jerry could be used for much more than audio. But on the audio front it does 16-stereo sound and full FM, AM and wavetable synthesis, with the whole package running at 26mHz, same as Tom.
The Jag' uses 2 meg of Fast-Paged Mode RAM, with the cartridges able to add an additional 8 meg using 24-bit addressing, though oddly only 6 meg of ROM can be used. Due to this small amount of non-volatile storage, most cartridges contain compressed data and decompress it at execute time -- depending on the amount of compression and the data being compressed, over 80 megabytes of data can be stored.
With the right code, all 5 processors can be made to work in perfect concert for amazing performance. However it suffers from one of the the same problems as the later Playstation 2 in that it has little in the way of buffering so as soon as data is generated it must be used so streamlining all of you I/O is extremely important.
Some argue that the Jaguar isn't really a 64 bit system because not all of it's components are 64 bit capable - however, by that marketing logic the Nintendo64 is only a 16 bit system as well. The main argument of the Jaguar being a 64 bit system is that the system bus is 64 bits wide, as well as the memory bank and Tom, where 3 of the main cores reside. Data in a Jaguar exists in the 64 bit realm and is them only chomped down to 32 or 16 bits when needed, versus some other systems where 16 and 32 bit data is doubled- and quadrupled-up when fed into 64 bit components.
Sources include the Atari Jagua FAQ and a former Atari Hardware designer I know.