There is many different ways you can do it, I think it really depends on case style though.. I've seen many with just 2, how many lights will you have that's a big decider, personally I am using 4/5 26W 2700k CFLs so I am going to have 4 80mm fans. If you use a 120mm for exhaust you could then use 2 80mm for intake. There's also passive intake, which basically is the exhaust fan doing all the work but you should use a larger fan if you go this route. Hope this helps!