The Dreamcast is not a easy system to Emulate, its not easy for most new PCs.
N64 Emulation depends on the software etc, as the CPU and GPU of most these Andriods should do the job, to me from Videos and comments it seems the YDP G18 N64 Emulation is not that bad, Xperia Play is better (thanks to higher speed and better CPU) i spoke to someone who said on the Play his Rooted and OC Play will play N64 at fullspeed for plenty of games when clocked at 1.2-1.5Ghz he says he sees about 10-12 FPS increase.
So if these open source consoles had ability to OC their 800Mhz CPU to say 1Ghz and had same Andriod as the play uses 2.3+ then we should see N64 Emulation as very well.
As far as Dreamcast i think its a bridge to far for the current devices not if you want 40fps+