the n64 had a clock speed of something like 94mhz which is pretty easy for the PIII/Celeron 733 in the xbox to handle. whereas the ps2 has around a 300mhz clock. not so simple. old DOS games can be played but most are painful to even try, i got DOOM running once and wanted to kill myself it was so slow.
effective emulators:
64- surreal64
snes- xsnes9x
sega master system- xsms
psx- pcsxbox
dos- dosxbox