When looking for a case, the most important part is the power supply (if the case comes with one) and that should be your main concern. Most good quality cases do come with good quality power supplies, but a case with a 400 watt power supply for only $75 will get you a decent case with a not so good power supply. You can do as bailey suggests and buy the case and power supply separately (which will cost you quite a bit) or you can get a nice case (like an Enlight) with a good quality 350 watt power supply (which will power most system easily) for around $55.
Why do you want a 400 watt power supply anyway? With power supplies, more watts doesn't mean the power supply is better. You want a good quality power supply that supplies clean steady power to the system. Any instability on the rails will lead to an unstable system and even random rebooting. A good quality 300 watt power supply will handle most home systems that aren't used for gaming. For gaming systems with a potent video card, a good quality 350 watt power supply is more than enough.
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