I have no case fans in my box. Only the PSU fan, CPU fan and the GPU fan. I used to have a fan blowing air out but I removed it, as don't think it's necessary. My CPU runs at around 30-40 degrees Celcius and I'm not worried about it overheating (it's an Intel chip afterall). I think case fans are only necessary if there's a lot of heat-generating devices in the computer, you overclock something or there's a lack of sufficient airflow (probably with very large/tall cases).
__________________
CPU: Intel Core 2 Quad Q9450 Mobo: Intel DX48BT2 Memory: 8192 MB PC10600 DDR3 Videocard: PNY Geforce 9800 GX2 Soundcard: On-board SigmaTel High Definition Audio Hard drive: 1 TB Seagate & 1 TB Hitachi Optical drive: LG GGW-H20L (2x BD-R DL) PSU: Nexus 1000 Watt OS: Microsoft Windows 7 Ultimate (64-bit)