The fan sitting on the case bottom doesn't draw air into the case, it's only to mix the air up inside the case... it's a huge one, about the size of the old 5 1/4" floppy disks, and it blows huge amounts of air.
I added the big fan because of the unique airflow in this case... since the power supply fans push cool air into the case at the top, it tends to create a 'blanket' of air that pushes the hot air out the front bottom of the case. I noticed that the bottom part of the case was always warm, so I added the big fan to mix the air up (I was worried about overheating the components on the lower areas of my m/b, especially the video card).
Anyway, my hard drive cage hangs at the front of the tower, right below the CD/floppy cage. I mounted the big fan so it's half blowing on the hard drive cage, half blowing up toward the CPU cooler. I've got plenty of room in this case for airflow... it's a pretty big tower.
Eventually, I'll build the custom case I've drawn up... it'll be a sealed case that uses active cooling via a Peltier device, with thermostatically controlled case temperature regulation... it'll be completely dust-free and completely silent.