Thanks for your help bishopi5!
Does that mean that my PSU is acutally providing the graphic card with 72 Ambs? (12V1+12V2+12V3+12V4) combined as shown in the diagram in the above link?
The computer store guy told me that my PSU 12v is only providing 18 Ambs which is not enough to power the GTX680. That is why I got very confused !
And if I go with this current PSU I might damage the VGA or encounter sudden shutdown due to low Ambs supplied to the VGA!
Please advise, thanks.