What is the difference between 60mm, 80mm 120mm, etc.? Size.
CFM (cubic feet of air moved per minute), RPM, type of bearing, noise, expected life are other considerations. If you want to monitor the fan speed with the motherboard, get a 3-pin fan that will plug into the motherboard (instead of a 4-pin which plugs into the power supply)--if the motherboard supports case fans/fan monitoring.
I use 80mm fans for most cases and that is what most cases that have a provision for case fans specify. I am currently building three 1.4GHz Athlon computers. The cases and case fans will be exactly like http://duxcw.com/digest/Howto/buildcom/socka/1.htm. The CPU fans will be Thermaltake Volcano 5's. They will not be overclocked. Larry