thanks. I have been thinking of building a pc for emus in the living room but a dramcast seems more fun
Check into performance reports for specific emulators before you get too excited. Many emulators do not run at full speed yet.
I have DreamSNES but it runs slow unless you disable the sound (rendering the experience "no fun"). So I run a PC emulator for SNES instead.
I have a Sega Genesis emu for the DC too and it runs well.
NES stuff runs great (NESterDC I think).
The MAME Wow-Pack runs like crap. I believe that there's a better MAME distribution out there, but I have yet to try it because I have a PC-based MAME cabinet anyway.
I tried a GameBoy Advance emulator when I first got the DC and was running amok trying emulators. It was unplayable.
Point is, do your research. DC emus are not perfect but it's a very active community and the situation changes regularly. Given that you are thinking about buying a DC just for emulation, you should know. I'm sure that lots of people here or at the sites I pointed you to can tell you about specfic emulators and which versions to get.
Then again, everyone should have a DC anyway so go for it!