well, it's easy...
the more ugly the case, the cheaper it is

starts with a beige 4 units box which can use a standard powersupply and is generally identical to a regular (tilted) tower case, including bolt positions etc.
Your attention should be on the depth of the case, in particular for 1 and 2 unit boxes, which can be over 30 inches...
these also require special powersupplies and PCI extenders, so I wouldn't suggest them for a 3 card setup.
I have one of those 'beige beauties', 20" deep, made of solid steel and it seems to absorb heat and noise quite well - it's heavy... more than one would expect a first glance.
Mine has the 'standard' harddisk cage, but there are also (better) versions where the disks are mounted deeper inside and right in the main airflow of a case fan.
cheers, Tom