80 wire flat cable is the cheapest and probably best for 99% of the system out there. If you must rely on round cable for cooling, then the air flow rate thru the PC is not adequate. In general, a larger PC case provides better cooling and reduces localized hot-spots.
Use HD TUNE to obtain the temperature of your HD. If the temperature is above 38C, then you MAY need to increase the cooling capacity of the box.