A top mounted fan is a good idea, really helps with exhuast, especially that little nook between the PSU and 5.25'' bays where hot air can really stagnate. I wouldn't go nuts, 1 is fine, leave space for a second in case you think it's absolutely neccessary. With a 120mm fan, the PSU fan, and a new 80mm top fan you have more than enough exhuast. Think about how you're getting cool air to the CPU and GPU now. The 80mm side fan is a good start. An 80mm fan in the front by the HDDs won't really do much to help the GPU or CPU, but a quiet, low RPM intake fan isn't a bad idea up there to keep th