i just tested Dreamcast and PSP on my RPi 3A+
PSP does not start (because of too less RAM, as @Bkg2k already mentioned), but Dreamcast runs fine! Slightly better than on my RPi 3B (without +)
In my eyes the RPi 3B (and i think 3B+ also) is too slow for PSP Emulation anyway. Most popular and more demanding Games (Gran Turismo, Grand Theft Auto, God of War for example) do not run good.
Only a few less demanding games and PSP Minis can run on RPi 3B(+), so if you want to play PSP games on the go, better get a used PSP on the second hand market (you can find one for 30-50€, at least here in germany on ebay and local market ebay-kleinanzeigen).
And also many Dreamcast games do not run perfect on RPi3 (the same with N64).
If you want to play more than 90% of the game library of certain systems without issues, i would recommend RPi3 only up to SNES/PS1.