http://cryptoyeti.com/ethereum-mining-gpu-cant-allocate-the-dag-in-a-single-chunk/

This post explain what does those settings, and as they state putting GPU_FORCE_64BIT_PTR=1 is good to put if you game at the same time as you mine (possible with process priority super low) because it will open your whole GPU memory for 64 bit adressing, if you do only mining, it's better to put it @0 because it could hurt hashing speed otherwise, well have a good read