A lot of this depends on two main factors:
1.) The ambient tempurature in your room
2.) Air flow design in your case.
If the open bodied cards simply pulling cool air from outside and dumping it in the case, it'll just build up and stagnate in the case unless it has a clear channel to exit the case (even then it'll still heat up the case without any active exhuast). Usually a case fan at the front of the case will help (if the cards draw air in from the PCI slots at the back, make it so the air flow is pushing air out of your case). 120mm fans are usually the best for this type of situation, but look up the mount sizes on your case before you buy anything. Hope this helps.
My motherboard is running between 60C and 63C while mining.
It is an ASUS Sabertooth X58.
I have two OCd 5850s that run around 75C.
I'm wondering if this is simply because the (open bodied) cards are throwing off a bunch of heat inside the case, or if it could be something else.