How to Control Computer CPU Temprature ?

My Computer has Intel Pentium D CPU of 3.00 GHz with 2MB catch and 1GB DDR2 ram and my computer is just getting hot and hot.. CPU processor is running at about 75 deg Celsius and The motherboard is running at about 50 Deg Celsius. It is connected with Original Intel Heatsink and Processor Fan with Arctic MX-2 Thermal Compound and a fan in the ATX cabinet. I think this temperature is really high. Currently it does not have any Extra Fans attached. Should I attach any extra fans ? How Many ? Please suggest me some ways to lower the temperature and also note that all the figures given at the top are at no load......