For Crossfire, if possible, get an exact brand, model, and version of the card you have.
Be *sure* you research the power requirements necessary for running both cards.
Upgrade your case fans or add more fans to increase positive OR negative case airflow that you decide upon.
Experiment with manual gpu fan speeds to get a temp and noise level you can live with.
Adding a 120mm PWM fan with Velcro (very secure mounting method) to the outside of the case in the video cards exhaust area (where your connection to monitor is) will directly assist in removing hot air from the cards and the case.
Lower ambient temperatures increase cooling performance.
Read tfm for your motherboard.