Btw, emulators on a dreamcast means the only things your gonna get are a good VirtuaNES emulator, and I hear a decent gameboy emulator, all of the other emulators are outdated and very very very slow. DreamSNES still runs at around 70% last time I checked. If you really need the mame experience without the big cost, just get a crappy computer on ebay, take out the motherboard and the hard drive and mount it on a piece of wood, It's somewhat small and will have a lot more power than a dreamcast.
Btw, the reason why you put a diode on the dreamcast ground is because the dreamcast ground will literally suck away 12V from any other source that is connected to the ground, Why?? I don't know! but I do know that if you have an Ipac and a Dreamcast connected, the DC will be happy, the Ipac will not be, so the diode prevents this sucking of power, and both will work happily, although don't run em together, one of them might send some kind of power to the other, and zap, there goes both your interfaces.
Also, emulators on the Dreamcast use the joystick, the only things that are keyboard only are the old verisons of Quake, and a few converted things like that.
-Chu