The best option to cooling is water cooling. If you aren't into that, good airflow in a case. Video cards tend to dump hot air out the back of the case. They will still get hot though. An NZXT Phantom already has 4 fans; 2 of which are on the side panel. that should be enough to cool your cards sufficiently. You can add another 200mm fan at the top if you want to.
Wow, some people just give very WRONG advice, then people beleive it and mark it as the best answer! Shame on you