Actually..... there is a DC emulator
http://www.zophar.net/dreamcast.html
It was a project started I forgot how long ago, but fairly recent (within the year) and it runs commecial games at fair speed..
As for PS2 emulators, there are some, but they don't play at full speed like maybe PS1 emulators
http://www.zophar.net/psx2.html
those are your options!