Lets take this one step at a time. Right now, I can run some SNES games on my DS with minor slowdown and graphical glitches using SNEmulDS. This is about as far as emulation on the DS has gone. Heck, N64 emulation still isn't running at fullspeed on the PSP, which happens to have a 333MHz core. The fact of the matter is, MHz aren't really a good speed comparison for CPUs across architectures.
The ARM7+ARM9 combo on the DS running at 33 and 67MHz respectively is not the same as a Pentium Pro at 166MHz, or a MIPS R4000 running at 333MHz or whatever the PSP uses. When people talk about emulating hardware they usually say that the machine doing the emulating must be running somewhere between 5 and 10 times as fast as the target machine. At its best, the DS is roughly equal in power to the N64, but completely different in architecture.
So is N64 emulation on the DS possible? Sure it is. Somebody could probably make a Dreamcast emulator on the DS if they figured out some way to use the flashcart or SD card as some sort of cache. But it would be slow. Terribly slow. So slow that the entire thing would be essentially pointless. So that's pretty much it for N64 emulation.