The ATOM is certainly powerful enough for the classics, and the classics are what are really going to shine on a cocktail machine anyway, since cocktails were really only around for a short time.
I had a VIA 1GHz motherboard in my cocktail machine, and it easily ran every single cocktail game that MAME supported. ATOMs are even more powerful.
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That said, if he wants to put a flat panel in there, now we may be talking about higher demanding games, and non-cocktail games, then all bets are off that the atom is going to do the job.
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Considering there is *plenty* of space inside a cocktail machine for a standard ATX motherboard, there's really no reason to go with a niche product like the atom board. Take that money and spend it on a fatter CPU and you'll have the possibility of taking your cab out of the classics only realm.