the number of fans..there placement and the size of the case and how far you over clock your cpu and the number of gpus and the size of the power supply all can make or break a gaming rig. small cases and larger gpu wont work. most gaming rigs use full size or large cases. for cooling most gamers will use a sealed cooler or a larger brick air cooler. (the brick coolers are fine if the pc not going to be moved.) if your going to lan game a lot look at the sealed units there lighter and less change of them breaking a mb or cpu if pc moved a lot. the newer gaming case have top grills and lower grills in the case to have cold air pull in from bottom and hot air out the top. right set up is front fans pull air in over the drives and cpu.
top fans blow hot air up and out. lower case fans if there room for them pull air in. with case fans using a good fan controller and good fans will keep the noise down. muilt fans running at 100 percent will drive you up a wall.
with cases and motherboard most good gaming mb have 2-3 slot spaces between two main video slots.
try not to jam your gpu so there touching each other try leaving a air space.