This:
http://scummvm.drunkencoders.com/
It only works with specific games like Lucasarts adventure games, but it does work pretty well!

how compatable is it?
what FPS does it run?

Could not find any PCE/TurboGFX emulator at all at Emulators for NDS.

@skinym3: I do not know if there is one that is only for slot-1, but have not found one and I really am looking for it.

SuperCard SD slot-2 used to emulate PCE natively but that might have been through GBA
I don't think there is a PCE emulator available for the for slot-1 cards.

I think that if we would put together SLOT-1 flashcard and some SLOT-2 as RAM memory like Opera pack it would be possible more or less but dunno if smoothly

Is being developed:-
http://www.dcemu.co.uk/vbulletin/showthread.php?t=275678
But for now it's not open source so you can't compile your own games into it yet, but from the demo I played it performs really well.

That is a x86 emulator, a PC engine is a console, but still that was a good thing to show.

He means a PC Engine emulator (AKA Turbo Graphx 16) not a PC emulator

there is a PC Engine Emulator you can use on the DS called PCEAdvance but you need a slot 2 to run the emulator since it's designed for GBA. However to run games in it you need to use a tool called PCEA/PNES Builder(or you can use the builder that comes with PCEAdvance) to insert roms into the emulator .gba file, you will also need PCEToy32 to decrypt most USA roms since most of them are indeed encrypted and PCEAdavnced cannot read encrypted roms.

Here are the links:
PCEAdvance
PCEA/PNES Builder
PCEToy32