Originally posted by: Navid
Leaving the case open will lower the temperature on components that have a fan on them like the CPU.
However, it may increase the temperature of the components that do not have a fan on them like the hard drive.
The components with no fan rely on an air flow through the case for cooling. If you leave the case open, there will be no air flow. At least, there will be no controlled air flow.
Your hard drive or your chipset (if without a fan) or your MOSFET voltage regulators may end up in a dead spot (no air movement at all).