I usually focus on a lot of input fans and let a combination of the video card exhaust fans and passive vents handle the output. If you have a lot of air coming into the case you will have a lot of air coming out.. The cases I'm using now I have 3x 120mm + 2x 180mm input fans and only a single 120mm output (near the CPU) + all the video card fans and some passive vents.
If you can get a high air flow for your intake fan it could help to add vented slot covers for the expansion slot covers between the cards (or just remove the slot covers) this can help remove some of the hot air from around your cards.
Silverstone vented slot covers:
I agree with the adding fans in the side panel blowing into the card intakes.
Also pay attention to cable clutter within your case.. try to eliminate or reroute cables which may block airflow.
Higher grade power supplies usually have modular cables which can be removed if not needed helping improving airflow in addition to the power savings from the higher efficiency.
If you can get a high air flow for your intake fan it could help to add vented slot covers for the expansion slot covers between the cards (or just remove the slot covers) this can help remove some of the hot air from around your cards.
Silverstone vented slot covers:
I agree with the adding fans in the side panel blowing into the card intakes.
Also pay attention to cable clutter within your case.. try to eliminate or reroute cables which may block airflow.
Higher grade power supplies usually have modular cables which can be removed if not needed helping improving airflow in addition to the power savings from the higher efficiency.