RE:
I'm sorry, I wasn't suggesting you were thinking of using USB. I was thinking a long the lines of what the pictures suggested: Essentially a group of connected Mac Minis.
Okay, just to ellaborate a little, here is the sort of connection I'm thinking of. The underside of the mini/brick would have a little flap with a slot pcb thingy that connects straight into...
(forgive the technical inaccuracies)
...Actually, the GPU could more easily just be on-board with no bridge connector necessary. This would be more optimal as far as waste is concerned because onboard GPU's share the Central Processor and System Ram anyway.
One top or bottom machine would be the "master" and all the others "slaves". All the primary connectivity will be assigned to the "master", but also expand available usb and FW ports to optimise further and lessen resource waste.
I've actually been kicking this little idea around for a while. What with multi-core processors, multiple GPUs, DDR ram, HDD arrays... Every PC component has divided to allow further expansion and therefore Every PC component could perhaps be extended/upgraded in this way.
I made that mock-up as a silly little "perhaps" idea, but the more I think about it, the more I think it could happen. If a company can mass produce and turn a profit on each cheap stand alone machine, then imagine how many they could sell. Multiple machines to single users ...and imagine the bragging rights of having the tallest stack of "bricks"!
Also, this mass production could mean that single module users would be able to pick one up VERY cheaply ...At a price that just wouldn't be possible if it was a singular product.
A very appealing little machine for many many light users at £165/$300. A low-end "PC" market killer product and it covers high end too.
The one reason I can see that it may not be profitably viable is "Universal product Capitilisation" (I just made that term up

) ...The way that a (for example) GPU vendor will cripple the same product at different levels to capitalise on all ends of the market. Therefore a product like a graphics card that costs roughly the same to manufacture can be sold at £100, £150 AND £200 and getting the maximum profit margin from every class of customer.
This sales model is used a lot by apple. imacs, ipods... The higher model always has a much higher % profit margin for apple
...With the above model, the profit margin % is always the same no matter which demograph is buying. Might work that way, might not.