Having an open case can disrupt proper air flow. What does the side panel of your case look like? What model case do you have? The open case is fine if the room is seriously cooled by the AC but it doesn't pull the heat off of some components as well as you might think. With proper wiring, the pathways within a computer can take advantage of having cooler air draw in from the lower portion of a case and create a jetstream, directing the air through the computer, over the components, and exhausted out the back. Of course, some people mod their cases (I did) and put fan blowholes at the top (for exhaust since hot air rises) and on the side (as an additional intake fan that blows cooler air directly on the CPU and GPU). Also, are you using stock cooling on the CPU and GPU?