A Turing-complete 8-bit computer can do exactly the same as a Turing-complete 16-bit or 64-bit computer. It will probably take a bit longer to do so, however. (Apparently, somebody has written a version of the Linux kernel for the Z80 8-bit CPU. I hear it takes about 3 hours for a login prompt to occur.)
We have used roughly a decade for each step along the way from 4-bit, 8-bit, 16-bit, 32-bit and lately 64-bit computers for domestic use.
I assume that within the next decade we might even progress to 128-bit computers, though I am doubtful about that. The Law of Diminishing Returns means that the extra costs involved probably preclude that happening, after all we don't do things much differently with our 64-bit computers than we did with our 32-bit computers.
