Some things to look for in a case.
Some come with power supplies, some don't.
The better quality cases have better power supplies.
Try to get at least a 350w ps. Anything less will be too small. The bigger the better...but, a high quality ps with a lower rating is better than a cheap one with a high rating.
I have built computers with 200.00 cases and I have built computers with 25.00 cases. The 200.00 cases are far superior to a 25.00 case, but when you get right down to it, it's still just a box to hold your stuff. So it depends on your intended use for your computer. If you are going to set it up and never move it again, any case will do. If you are carrying it to LAN parties, college, etc, then you need a quality case (such as a Cooler Master Ammo 533 case) that will take the abuse.
The size of the case is important too.
An ATX case will hold a full size ATX motherboard and more stuff.
A mATX case is smaller, requires a mATX motherboard and will hold less stuff, but takes up less room.
A new format called BTX uses a BTX motherboard and PS. (Not too common yet though.)
Other than that, the skys the limit.
PS-Check your private messages.