Case mods can be tuff at times depending on what you are trying to do. Most cases see one opening for a front fan already designed into the way the case itself is layed out as far drive bays, front power.reset buttons, and any usb, audio, or fireware and eSata ports.
The 900 was made to be flexible with either two drive cages each seeing a 120 on front or removing one of those to six 5 1/4" drive bays instead of the three seen. What's nice about the 900 is putting the hard drives in the bottom cage and nothing in the upper one just for the additional intake fan.