Pc engine... I love this gear ! I got a Nec GT turbo before the Gp2x and it was, before Psp & Gp gears, the best handled on the market (my point of view ).
But it still a 8 bits machine for the processor part (a 65CO2 -> 7,2 MHz).
The thing in the Nec gears that was 16 bit was the 2 graphical co-processors.
The Pce emulation was the first thing that motivated me to buy a Gp2X.
EDIT : I gave a try to Atari St emulation last night. It's really amazing. I played many demos on it and it was almost flawless.
I hope that Amiga demos with Uae4all could work as good as that, one day.
Sega Genesis/Mega Drive: Fullspeed, nearly no glitches
Amiga: Still a little bit slow, but nethertheless GREAT!
Atari ST: Nearly flawless, fullspeed!
SNES: Runs great with frameskip 1, mostly playable for most games. Will see improvement in the future, just check the newest hack.
NeoGeo(CD): Runs nearly fullspeed with sound, me thinks in the near future it will be fullspeed @ 200Mhz. Definitely the reason to buy a GP2X!!
Oops, sorry...
Okay, for the GP32: MegaDrive/Genesis is just great, most titles play with fs0/1 @ 150Mhz (my GP32 can't go faster...
Atari ST runs great (just tested too little games to give an overall opinion...) most titles I ran were fullspeed.
For SNES I only played DonkeyKongCountry which was playable (I think somethin like fs1/2) with sound until it came to a level were transperancy-bugs came in and it was unplayable.
Overall, the GP32 is for me just fine, especially because the batteries won't run out in less than a minute as on the GP2X...