One could say the Atari Jaguar is 64 bits or 16 bits (since it's powered by a kinda 64 bits Tom GPU and a 16 bits MC68000 CPU
Quite the same can be said about the Dreamcast, that proc ranges from 16 'till 128 bits.
Rather look at the amount of polygons per second, MIPS and special graphic features (like texture filtering, and so)
fact is, both systems (+jaguar cd unit) I got in my posession :-D :-D :-D