I am curious as a total novice why chips and why now? As you said ARM is cheap. For price to performance I don't see why there aren't multi-motherboards coming out.
Someone made a 10 raspberry pi MoBo which if it is cost effective would be easier than a cluster. I think it would be awesome if that was scaleable but I am sure there is a reason. It would be cool and save space making a cluster out of 4 of those boards rather than 40 rPis
Toy SoCs, like those used in almost every single-board computer, have bad I/O, which makes them poorly suited to general-purpose clustering. Also, general-purpose clustering is not something people strive to bother with. Especially not at a performance level that is as low as a bunch of Raspis.
Someone made a 10 raspberry pi MoBo which if it is cost effective would be easier than a cluster. I think it would be awesome if that was scaleable but I am sure there is a reason. It would be cool and save space making a cluster out of 4 of those boards rather than 40 rPis