The space between a motherboards southbridge and the videocard is usually less than a centimeter. A videocard running at 90+ C will heat up the southbridge a lot.
I actually tested this, I noticed how the motherboard temperature is always around 30-40 C even if the CPU and memory are under full load. Yet as soon as I started stressing the videocard the motherboard temperature rised by almost 15 degrees. I must note however that this may vary depending on where the temperature sensor is located on the mobo, but in the end it’s still heating up your motherboard even if your sensor is located somewhere else (like next to the memory).
Anyway, discussions about computer temperatures usually end up turning into a shitfest, what you do with your computer and what you think of the temperatures is up to you.