The N64 uses an R4000-derivative CPU at ~96 MHz, the PSX uses an R3000 at ~33 MHz. The PSX is still not full-speed on the Gp2x, though that will be attained in due time. The R4000@96 can be pulled off on the GP2x, but it's going to take, as was said, a very fast ASM core. Porting over some SDL-compatible N64 emulator written in C is going to be very very slow, and would just replace "Is n64 possible" threads with "How do I over clocked gpx2x to 900 MHz to run n65 gaems???//"
As for the Saturn. That has 2 SH-2s at ~32 MHz each. That's doable, but it's more stressful than the PSX, and there's a much larger obstacle. The Saturn was an engineering nightmare, its implementation of a master/slave CPU, 2 VDPs and lots and LOTS of helper chips, even a 68000 just for audio, made it a very powerful but VERY DIFFICULT system to program for .. and probably even TOUGHER to emulate. There is not one 'near-complete' Saturn emulator available for PCs yet, and the existing ones are not only buggy, but also rather inefficient and can run dog slow on even a modern CPU.
Short answer, keep waiting and see.