Why would you buy another fan? Are your other temps high? Your current CPU idle is good, not high, but you can lower it by getting a heatsink for your CPU at almost the cost of a case fan
An intake fan will lower temperatures.
Also with only 2 exhaust fans,one in power supply and the other on the case back, he will have a very negatige air pressure in the case. This will pull dust in every crack and crevice in the case. Including floppy,cd,dvd,BR drives,ethernet ports,usb ports etc...