Keeping the side panel open is not going to provide cooler temperatures.. It might actually increase it.. The primary reason being, uneven airflow.. Why do you think so much time and effort is put into designing cases..!! A good case has properly directed airflow scheme.. The cooling efficiency will depend on various factors such as design, type of fans used, component setup etc.. The Antec 902 is pretty good in all those departments and is one of the better known cases based on air cooling.. Looking at your rig, it should not have any problems in keeping it cool (without removing the side panel of course)..