Cases are chosen on many criteria, and as it's going to be your case you need to think about these things and come up with an answer that is right for you (it's a lot like chosing a mouse in that what works afor Joe might not be right for Sam). Consider:
1. The form factor. What size motherboard will you be using? The mobo has to fit in the case - you can't get a ATX board into a mini-ATX case. You can get big cases that will take pretty much anything, but maybe you don't want a tower. Will it sit on your desk or under it? They come in tower and desktop (more commonly called home theatre now) for on your desk and still handle full size ATX boards. Will you want to haul it to places or will it always sit at home? If it's to travel, seriously consider the weight of a prospective case and avoid plastic and easy to scratch finishes. Will you be fitting a lot of components? Towers have lots of space, but if you aren't fitting more than 2 opticals and 2 hard disks then maybe something smaller will suit you.
2. Extras. Do you care if the case need you to use a screwdriver? Cases can come screw-less or the basic screwdriver option. Do you care about lighting or watercooling or showing off the inside of the case? You can get cases with windows, with fittings for watercooling (for easier installs) with cabling systems that run behind the mobo for a better display. Perhaps you would want a certain minimum or USB ports on the front or don't want headphone and mike jacks? Maybe you want an inbuilt fan controler or a DVD drive? These options while not necesarily as important as other considerations will help you weed down your choices.
3. Cooling. Are you going to overclock? You might need extra ventilation from fans or apertures. Does it have a 'cooling system' like a tunnel or zones? Do you need it? Will that be compatible with you cooling fan/water setup?
4. Looks. Do you care what color the case is? Do you want plastic or metal facias? Do you care? Do you want it to have a front door or maybe some kind of cover for the drive bays or ports? Maybe there is some kind of style that appeals to you or maybe you don't care what it looks like and just want it to do what it does or it's cheap.
5. Cost. Seriously, do you want to spend a lot on a case or would you rather spend the money on components? Maybe you want to fall in between? Set yourself a price-point and look in that range. More expensive cases generally have more features and a better finish, but they all hold your components together and control air-flow to some degree. The price runs from generic beige box for £10.00 all the way up to some fanless case for £500 I saw last week.
See, this is a very personal kind of thing. There are plenty of people who see the £10.00 case and think "that's for me" while other think about the times they cut their hands on unfinished steel and think they want to do better. Some people want to show off, while others will have the case sitting in a closet as a server, rarely to be seen. Keep the thoughts you have from these questions and then look at the selections of cases on some websites and see what appeals to you. Once you see something you like, look for a review or two and have a read, see if it suits you and your needs. You'll find yourself moving into some definite wants and needs and trying to find something that suits. If you are still stumped, then come back here with those specifics and I'm certain the good folks here will give you thier opinions as to specific cases or suggested cases.
. But your point is well taken and ill have a look at the 939 amd. I could do with the extra bucks! 
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