I agree about the airflow. You have basically 3 different size cases. Desktop, mid-tower and full size.
As a rule the desktop will be the size of the Dell, typically with 2 external slots for 5.5" ROMS and 2 for 3.5" floppy's and/or fan controllers.
A Mid tower will have 4 ROM bays and 2 floppy's.
A full size/full tower will have 5 to 6 ROM slots and 2 floppies.
The width and depth of all 3 will vary slightly, but the height will be the biggest difference. There will be differences in the amount of internal slots as well, so take that into consideration if you are planning something like a large raid array. A micro ATX form factor mainboard will fit into all three.
For just general computing and no liquid cooling or anything exotic, I'd say get a mid tower that has front intake fan slots as well as rear exhaust. Current hardware generates more heat, and good room for airflow as well as using rounded cables is pretty much a must.
The other thing worth mentioning is the power supply. With the exception of a few high end cases, most cases that come with power supplies, have PSU's that for the most part suck. Just get a plain case and buy a good power supply separate. Remember the power supply is the only component that has the capability of destroying every other component, so quality is a must. If you do decide to get a case with a power supply look at the PSU label (NewEgg to their credit usually shows a pic of the label) and make sure you have a minimum of 20 amps on the +12V rail.