Racks vs. pile of PCs

[Mark Hahn]

> One advantage that comes to mind, although it may not be worth $6000, is
> when you have 20 PC's stacked, according to Murphy, the bottom one will fail
> first. The rack mounted version will be much easier to service a failed unit
> than the stack version.

wire shelving units cost very little and often have an option for wheels.

I think the choice really boils down to the number of nodes you want.
chances are good that your floorspace is limited, therefore density
is an issue.  we want O(150) CPUs and can only afford ~2 more racks 
of floorspace in our machineroom, so we need roughly .5U per CPU.
further, with more than, say, 64 CPUs, you have to think seriously
about hardware failures.  that, in turn, biases you towards higher-end
hardware, since a generic ATX power supply is definitely not in the 
same MTBF range as a good rackmount one.

still, Dell/Compaq all make small uniprocesor desktops; for instance:

http://www.dell.com/us/en/biz/products/model_optix_optix_gx260.htm

3.6*12.5*13.9=625.5 cuin, versus 19*21*1.75=698.25 cuin for 1U.
wow, you can actually get 2.26GHz P4, 533MHz FSB, 512M ddr, 1000bT
all for under $1k.  3-year warrantee.