The way I'm doing it, that was recommended by somebody here, is to interface my arcade controls to a 3rd party playstation controller which can then be converted to usb or almost any console. In your case if you just want pc and dreamcast, you could interface the arcade controls to a dreamcast pad then buy a dc to usb adapter, or if you already have an ipac or something you could make swappable adapters. There is a guide around here somewhere that shows how to make the console adapters.
Good luck.