i do remeber some ppl used a broad band adapter for dreamcast, so thy do have adapters for it..
do u have a computer adiquitly capable of playing Q3 and unreal?
if so, i highly suggest using that instead, the graphics on the pc are much better, and there are a lot more servers to use... especally since thy turned off seganet, only servers for DC avalible now are custom servers(well for Q3 anyway, thats what i played it on).
what are your computer specs? most comps can play Quake and unreal, thy arnt graphic demanding games.