Yeah, as HT said, it's fine to moderately overclock. You are going to need it for some things to run well on your Wiz. And you might also want to UNDERclock it when you are playing less demanding stuff.
Just don't run it too hot for too long. I've never used a Wiz, so I don't know exactly how to go about overclocking it. Some of the Wiz emulators should have an overclocker coded into their menu that you can use, like FBA2x, choosing different clock speeds for different games.
Oh, and as for some emulators running slowly... before you overclock, look for a "frameskip" option somewhere in the menu and mess around with it, and see if that helps. I don't think the Wiz should be having problems with NES games. But I don't know. Maybe it's just the emulator you are using. I'm not really familiar with the emulators available for the Wiz.
What I do know is that the default NES emulator on the Dingoo A320/A330 is superb.
Actually, I hear it has a problem limiting frames or something, so it runs a little faster than it should, sometimes, maybe? I haven't really noticed any inaccurate speeds so far on the games I usually play. Then again I haven't put too many hours into it...
Now I'm rambling...