ExtremeTech has a thorough tutorial for picking the base components of your DIY home theater computer, the case and power supply.

They cover the variables you need to take into consideration for cases:
Form factor
Ventilation
Noise control
Materials and construction
Appearance

And power supplies:
Total electrical output
Cooling approach
Noise output
Power connections:
Electrical efficiency

The tutorial walks you through all the details and takes a look at a couple of case choices.

They wrap it up with a look at barebones package alternatives.

This is a great place to start if you want to build your own HTPC.

A Case and Power Supply for Your DIY Media PC