I assume you mean the case of the computer not the actual tiny cpu. It really depends on quite a few personal prefrences:
how much of an enthusiest you are
how often you upgrade
How many and what size of fans you want in the computer and cooling set up in general.
Also looks if they are important to you. I tend to stick to plain black cases with no lights or flashy gimmicks. If you are a frequent upgrader a tool-less design would be best
There really are many factors in choosing a case now that I realize it
[h=1]I would say it is worth it to have a PSU that runs as efficiently as possible. This is a good indicator of power supply component quality. Don't skimp on your PSU, there will be other ways to save money.[/h]
It all depends on personal preference. As long as your components will fit in it with decent airflow, that's all that matters. My personal preference is full sized towers for plenty of working room. And I want to like how it looks.