another consideration would be the heatsink and fan you are using for your cpu..if you are going to install a fan on the side i would set it up as an intake blowing air near your cpu.
It's hard to say..my cpu idles between 35 and 37...alot of factors come into play with that though.. like what cpu you are using, thermal grease and so on. I would be more concerned about temps under load. I personally have 2 120mm intake fans in the front..1 120mm intake on side..1 120mm...