If there is another card blocking the fan or is very near then your graphic card will get warmer. If it will overheat who knows. It's best to test that by runing a 3d app or possibly a game in a 3d windows and look at the nvidia control panel for the temperature.
Download this program I use it http://www.daionet.gr.jp/~masa/rthdribl/
Then opet up nvidia control panel or rivatuner to look at the temperature. Push it for 30min and look at the temperatures.
It's best to leave an open pci-e slot next to your graphic card (even two would be good) and make sure you have good air flow inside your case.