so i see that caanoo, wiz, dingoo can run a psx at a descent speed, but how come it cant run n64 witch has lower graphics or the dreamcast witch is just at the same, if there is such software i would like to know. Also i have seen the OpenPandora run n64 and seen no gp2x's run it, i have seen few psp's witch aren't made to emulate run sonic adventure 2 this makes me very confused.
And to further muddy the waters and answer the very original question:
The Nintendo64 has a 64-bit processor, running at close to 100MHz, and while it has a narrow 32-bit system bus, the fact remains that its a 64-bit system.
The Dreamcast runs a SH-4, which is 32-bit, with superscalar capabilities (two-way instruction handling), at 200 Mhz.
The PSX has a 32-bit processor clocking at about 35MHz.
There's a very skewed, but apt to this discussion, evaluation of whether one system can emulate another: that it requires the one system be 10 times faster than the emulated hardware. So, a Caanoo with an overclocked ARMv5 can handle PSX emulation, but N64 emulator would require something closer to 1Ghz to emulator properly (for reference, the Nokia N900 mentioned above running Mugen has a Cortex-A8 that can overclock to 1.15 Ghz).
As this discussion proves, there's a lot more to it than simple math, but at the very least its a baseline to work with as far as expectations of what one system can do.
*Of course, most games made for the N64 never took much advantage of the 64-bit architecture, and generally used 32-bit operations, since it was fast enough and detailed enough to provide for the graphics-processor, so maybe that point is somewhat moot.