parani :
but any idea, why does the pc turns off while gpu fan running at extreme speed at 2psu combination?
maybe the PSU is not supplying the power to the card in a stable and consistent manner.
about that Enter 450W PSU, most good quality PSUs have a efficiency of at least 80% . so 80% of a 450W is 360W. your Enter PSU could be providing less than that.
the card recieves it's power first from the motherboard i.e 75W. next from the PSU it requires around 45W. your motherboard is powered by the Enter PSU's 12V rail while your card is powered by the Dell PSU's 12V rail. maybe this is what is causing the instability.