I love the DC for it's emu abilities, but even still....
I highly doubt it would be possible to emulate sega cd. Like it was said before, to much for the machine to process. The dreamcast is powerful, but not powerful enough. The snes emulates a bit slow unless you turn off sound(but if it didn't have sound, I'd rather play on my pc) Nes emulates perfectly, as does gb. gba isn't getting much anywhere, and correct me if I'm mistaken, I think that people are getting places with playstation emu(other than bleemcast) If your looking for a good emu, I'd suggest checking out nesterDC.
http://www.dcemulation.com/ is a site for dc emu.