Oh no, not again!
We have three threads running now where ppl retardedly keep phantasizing about this topic over and over again.
Once and for all: The Neo is a damn 16Bit system with two processors/CPUs, one Motorola 68000 and one Zilog 80A, which, among other things, is responsible for some sounds, various x-tra stuff, etc.
Now, the Z80A is an 8Bit IC (read: has an address space with a width of 8Bit), while the M68k has a 16Bit wide bus, but the CPU (central processing unit) is the 16Bit M68k, the Z80A is just a co-proc.
The 24Bit shit stems from the early 90s and the efforts of some overenthusiastic marketing dudes at SNK. To successfully compete against companies like Sega (Master System, Genesis/Mega Drive), Nintendo (NES, SNES) or NEC (TurboGrafx/PC Engine), they simply added 16Bit to 8Bit and used this as their "weapon" against "pure" 16Bit systems.
So lemme' repeat that, there's no such thing as a 24Bit game console, never was, never will. It was just a made-up story to boost sales, that's all there is.