V-sync keeps you from seeing horizontal tearing in your screen when your graphics card pushes out fps numbers higher than the refresh rate of your monitor (typically 60)
so if you have 120 fps, on a 60hz monitor, you should turn on v-sync for a better gaming experience.
if you have 20 fps on a 60-120-240hz etc etc monitor, then it doesn't matter
I have v-sync turned on and still experience horizontal tearing. Also, is your maximum texture detail Medium? Please, WTF, with 4GB Geforce GTX780 card on 64-bit Win7 system? No way!