epsxe for the psx
its the current best working one out atm with many plugins etc
u can either d/l isos of the games and run them from ur hd, or burn them/use a real psx game in ur cdrom drive and run it from there
i got tekken 3 rom and its only 30megs
as for n64, there was a really old and good one called UltraHLE (its one of the first and quite good) u can d/l the roms from various sites across the net, they average ~20megs each (i think mario64 was around 16 while goldeneye was probably 30 or so)
ps2 emulator? yes there is but at the moment they are very primative (often lack of sound support and such) they also do not support as many games atm due to the encryption not being cracked for many newer games
xbox emulator... there are currently 2 cxbx and xeon as usual since both these 2 consoles are new, the emulators are currently very primative and do not support many games
for both of these u'll need the actual game on cd(or dvd?) because the isos would be rather large and i don't think there are many isos out there unless u get them off irc/torrent or private ftps...