Mastermind, what sort of money are you thinking of spending? There are cases from about £15 to about £200. They vary enormously in quality both of the metalwork and the power supply (where one is fitted). Also what sort of style do you want. Some are very ornate while others are quite utilitarian. You can get aluminium cases to dissipate the heat better as well as sound insulated cases to contain the noise from the fans.
Worth spending a little bit extra. Cheap cases tend to be very nasty and flimsy on the inside.
We have quite a few machine from a company called RM (research machines, big in schools). They have some of the most solid cases I've ever seen. Just about kills me to pick them up. I'd like to know where they get them from.
I like the look of the Chieftec cases but they are quite expensive and I don't think they come with a power supply.