Build a cheap supercomputer

Select the case for your supercomputer. This should be large enough
to hold all of the required drives, while still providing enough room
for air circulation to keep everything cool. Server cases are perfect
for this task since they are designed to hold large components that
likely produce a lot of heat. Keep in mind the form factor of the
case, which will determine what kind of motherboard it will
support. A standard ATX case will be compatible with the most
motherboards.

Screw the motherboard into place. This vital component should be
picked out with the processor(s) in mind. A supercomputer will need
a lot of processing power. Most supercomputers have multiple
processors, so if that is a desired feature, you will have to select a
motherboard with multiple processor slots and matching processors.
Another option is to select a motherboard that supports a single
processor with multiple cores. A quad core processor provides the
computing power of four processors on one chip.

Slide the processors into the sockets, and lock each arm down to
hold them in place. Snap the heat sink and fan over each of the
processors to help keep them cool. Each fan should connect to either
a fan header on the motherboard or directly to one of the Molex
power connectors from the power supply unit.

Install the hard drives. A supercomputer needs a lot of storage
space. It is possible to install several terabytes on a single system.
The largest limiting factor is budget and the motherboard's hard
drive hookups. Usually the motherboard will provide enough
hookups for four hard drives. If needed, a peripheral expansion
card that provides additional hookups can be used as well.
Install the rest of the computer hardware as you usually would.
Make sure there is enough RAM to support supercomputer tasks. A
memory bank of at least 4 to 8 gigabytes should be considered for
maximum multitasking potential. Ensure there is adequate airflow
through the case, and that all wiring is secured out of the path to
prevent it from causing restrictions.

Install the operating system. While a commercial desktop system
such as Windows Vista will work just fine, the full power and
stability of a supercomputer won't be realized unless a server
operating system is used. The Windows Server platform or a Linux
distribution specifically designed with servers and supercomputers
in mind will provide the most power from your hardware
selections.
(I must warn you you should turn the supercomputer off to avoid it drawing a lot of electricity.