First, it seems your PSU is damaged because it shouldn't make any sound other then the loud coming from its fan. When it does make weird sound, it means it isn't functioning properly.
Second, your mobo may have been damaged due to the 6950 incident. You need to understand that the trip of the circuit breaker may be caused by a short happening within your PC. There are no circuit breaker within the system so a electricity surge may damage anything connected to the PSU.
Third, it is hard to say what exactly is the problem as it can be a number of things. The best way is to bring it to a shop and have them cross check each component as they have lots of spare parts. Usually I will suggest people to try to find the problem themselves first, but a) your problem isn't localized and b) you don't take precaution when it comes to computer parts. A CPU HS may fall off or not assemble correctly by user, improper mounting happens and CPU has mechanism to prevent catastrophic cooling failure from damaging the system. That doesn't mean it is perfectly safe to operate a system without a HS. GPU HS are pre-mounted by about 10 screws, and it is not designed to operate without a HS, nor have mechanism for cases like that. Other that the HS, the thermal pasting is very important too as it will lead to cooling failure if no installed properly. With lead to the next reason.
Fourth, you moved the HS left and right and complained about its noise and high temp reading. Well high temp reading causes the fan to spin faster, but faster spinning fan doesn't necessary mean it is moving air, nor moving heat to be specific. It sounded like the installation of the HS is problematic which ended up causing your 6950 to fail, and you probably used the same procedure to install it back to the 5850, which again ended up with high temps and loud fan. To ensure something will die, you run futuremark and SC2 over and over again, which are programs known to kill video cards with improper cooling.
Fifth, I am assuming you have put thermal pasting between the HS and the chip, but I also assumed that it was not done properly. I have heard people use "tooth paste" as thermal paste, so I can't be surprised anymore.
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