Currently, those system you're requesting cannot be emulated, at least, to the extent that you want. There are emulators in development for PS2, XBox and NGC, but none of them can actually play official games (although, if I'm not mistaken, the NGC one is getting fairly close). Most can only, atm, handle the systems BIOS, and perhaps play a homebrew game or two. If you're really interested in the development of these emulators, check the NGEmu forums
as most developers post there about progress. In short though, don't expect to be playing FF XI or Super Mario Sunshine on your computer anytime soon. PSX, N64 emulation, however, is VERY big and works quite nicely, I hear Dreamcast emulation is decent as well.