I don't remember the exact details on why the N64 was believed to not be emulatable by the Playstation 2, but I think that it was because of the sort of power that the N64's hardware has.
Originally Posted by willmon22
In that case, it would be like the 32x extension for the SEGA Genesis/Megadrive - each of it's processors may be weak in comparison to the Playstation 2's 294MHz R5900, but when combined together their combined power is really quite great.
As for emulating the Nintendo DS - if it cannot be done on the Playstation Portable (With it's slightly more powerful MIPS32 CPU), it surely cannot be done with the Playstation 2.
There is a project that involves making Nintendo DS emulation on the Playstation Portable a reality, but it hasn't shown performance and compatibility that is compatable to the original DeSmuME emulator.
Unmodified SCPH-77006 with SM 3.6
SCPH-39006 with M-chip modchip, SCPH-10281 NA and refurb Seagate 80GB HDD
SCPH-10000 v1.00 with SCPH-10190 PCMCIA NA and SCPH-20400 HDD unit