REBIRTH: sounds like you might have put your 'main' PC in your cab!
I don't think the overclocking cheat will help in this case. This only applies when you are running eg Metal Slug at full speed (60FPS). You will notice at times some slowdown when loads of stuff is happening / massive sprites blowing up etc. This slowdown would have happened on the real NeoGeo machine, so is emulated correctly.
To 'overclock' the emulated NeoGeo hardware, I believe you can use cheat.dat and activate the cheat from within the MAME GUI. From my understanding you have to do this each time you play the game tho.
To be honest, I have never done it so I am not 100% sure how it is done. It might be worth having a browse throught the 'EmuAdvise / Help' message board at MAMEWorld - I have seen this topic mentioned a good few times before. If you can't find it, ask and one of the regulars will no doubt help you out

Just to confirm, this will not help the speed of the games if they are not already running at full speed In fact, it could make it worse.