Running a pair of GTX 780 in SLI mode will get the cards hotter in excess of 79 º when playing games like Crysis 3 for example, this game in particular uses 99% load.
The GTX 780 stabilizes at around 866MHz with the fan set to 70% anything more will increase the fan speed.
It is important to note the setup you currently have and most important is the type of case you have, it is recommended to have a more “open” style case, therefore you will get the best airflow through the case.
After performing many experiments with SLI and temperatures. To highlight and to summarize there are two main factors:
1) Distance of separation of your cards: strongly influences (as much as 5-10 ° C difference between the two) depending on how well you have refrigerated your box.
2 º) Cooling your box: The key is to have direct fresh air to the fans as much as possible, to get that airflow to the rest of the cards. The best way to cooling video cards is to pump as much air to the card’s cooler fans and out to the back of the box. This key to the survival of you high end GTX780