I have two 80-mm fans blowing air into my mid-tower case and a 120-mm one exhausting air. There are two others fans blowing air onto the video card and the HDD's (plus the P3 CPU fan and the PS fan). With all that, when I take the side off the case the temperature drops significantly !!
So in the summer I leave the case open. Keep in mind that if you do this, you will have to clean the inside of the case well from time to time. I put a piece of "chicken wire" screening over the open side for protection.