Welcome to the forums!
The 3870x2 requires a minimum of a 550w PSU with two 6 pin PCIe power connectors.
If you are interested on overclocking it, it is recommended that your PSU has a 6 pin and an 8 pin PCIe power connector.http://ati.amd.com/products/radeonhd3800/requirements.html
It sounds to me that you PSU is not capable of powering your system.
Could you please post your full system specs including the brand, wattage and +12v amperage of your PSU and any overclocks?
Also, in the future, please start a new thread in the proper section of the forum when you have a new question.
Most of the time, you will not get any help on your issue when posting in an unrelated thread that has been dead for three months.