I'm currently using a rack case and have been using a tower for years.
It's not the case that makes it quiet, it's the compenents.
Graphics card and Power supply are the noisest depending on what models you get.
You can add a ton of fans, grab a zalmann fan control and turn themall to the slowst setting which will be near in audible, but if you have a noisey psu or gfx card then you're still going to suffer.
For psu I always go with seasonics as they are rock solid and still very quiet. You could go with a water cooling kit for the rest of your components, including a gfx card if you're brave enough to mod it yourself and grab a couple of silent hd housings aswell.
If your rack is ina machine room you have no problems anyway but I'm sure a whole thread could be made just on the individual steps needed to make a silent pc.
There'sa nice lil site called Quiet PC-Quiet PC Hardware-Quiet PC Components-Quiet PC Zalman Products
well worth checking out.