Here is my gaming PC that I built 2 and a half years ago and only had a few upgrades since then. One of these upgrades being the Second AMD 6870. This still runs modern games at high/ultra. The CPU is overclocked to 4.4 Ghz. The case is an Antec eleven hundred which provides plenty of airflow to both GPU's thanks to the 2 intake fans with a third fan just in front of the top GPU.
Any questions regarding this build I will be happy to answer.