I have done game emulation on an RPI2 and RPI3, i used emulationstation / retropie and found games above 5th generation (ps1, n64, dreamcast) do not play well. Once the RPI has a better GPU i expect that to change. But for GBA/Genesis/SNES and others its alot of fun and very nostalgic.
For these systems you need to overclock the Raspberry, Psx runs very well, N64 only some games but I have no complaints, they work well, in Dreamcast is very limited emulation but some games work decently. If anyone has doubts in the retropie wiki you can check the listings of N64 and Dreamcast, Psx is not listed because all games work very well. https://github.com/retropie/RetroPie-Setup/wiki