2x7990 or 4x7970

[epixoip]

I set manually the speed to 100%. But I see that 100% starts to work when the temperature reaches ~70 degrees and above

this isn't right. even if the temp is 20C, if you set the fan speed to 100%, the fans should immediately jump to 100%. there shouldn't be any delay.

I think over the decision to place the cards in a single small case with good ventilation and plenty of space between them (There is no way to place cards outside of the case, I want a universal safe option). Is there any ready-made solutions for this? Maybe a small case like microATX? But there are many questions - is there be enough length of risers, does it help or not, ...?

no no, you have the completely wrong idea. you cannot have all that heat in a small case! you need a bigger case with more fans and more airflow. or better yet, you need to build a custom open-air chassis.