Re: overclocking GPU
Ambient temperature is important too. In the heatsink fins, hot moves to cold, so it can never get below your ambient temperature. If your blowing 40c air with the fan ... it can never get below 40c as an example.
That I guess why you'd see differences between systems as you would have to take the entire chassi and cooling into consideration.
Some easy steps can be taken though, such as cable management. Make sure you got a good airflow in your case. Intake fan in the front, and exaust in the back. You might want to have more intake air then exaust to get a positive air pressure in the case to keep dust away.
Buy a bigger case, directly related to above and it can make huge differences.
Just the other day I moved from my old Nobless Mirror case (midi Atx) to a TT Kandalf full tower. Gfx card temperatures was lowerd with a few degres just by moving to a better ventilated case with more room in.