Thank you all for your answers.
When I was trying to fit the 8800 GTS, I think that the chipset heatsink moved. The thermal paste must have been all dried out. BTW, I have the
Blue Zalman Silent Northbridge Heatsink ZM-NB47J :
I'll try removing the Chipset heatsink completely tonight to see if the chipset was damaged. I hope it's possible, I always had some problems when working with the push pins.
I can also try the RAM swap but I already used that trick 2 years ago when my system was unstable. I doubt it would work again as I risk returning in the unstable configuration. Those boards are really sensitive ...