The compatibility and ergonomics which should be taken into account when deciding what parts to buy.

Compatibility
Youll want to make sure that all the parts you buy work together without
problems. The CPU, the motherboard, and the RAM in particular must be
compatible with each other. Check the motherboard manufacturer’s web site;
most will list compatible RAM and processors. Often quality RAM that is
not on the approved list (but is of the proper type) will work anyway, but the
manufacturers list of processors should be rigidly adhered to.
Again, youll also want to make sure that your operating system supports
the hardware you choose. Windows is supported by almost everything, though
watch out for older components if you’re planning on using Vista. If you have
any interest in running Linux or another operating system now or in the future,
buy parts that are supported by that OS. It is also worth checking around the
Internet to make sure there is no history of your chosen components clashing
(e.g. certain combinations of hardware causing instability, crashing, etc.)

Ergonomics
Ergonomics is the science of designing things so that they work with the human
body. This is obviously important when choosing peripherals such as a keyboard
or mouse but should also be considered when selecting a monitor, and especially
when setting up the computer for your use. If your wrist hurts or youre getting
a crick in your neck, look at the physical setup of your computer, check your
chair height and posture. An ounce of prevention here can avert troublesome
repetitive strain injuries.