The Dreamcast won't play alot of the newer gamnes, but handles classics like a champ. Im pretty sure you're gonna need older ROMs, though. They update ROMs as newer versions of MAME come out, and I think MAME for the Dreamscast is only available for like v .37 or something like that. At least it was the last time I checked (a very long time ago). Plus people don't have much of an incentive to get newer games working on a Dreamcast since they won't play, anyways. So as far as I know, development has come to a stop for that platform and you may need older ROMs to be compatible.
(This was all the last time I checked a long time ago, though.)