Vram gtx 570?
 

I have a gtx 570 classified in my system, and i am thinking of adding another one now that i have a job. However, i am not sure if it will increase the performance of my system (gaming wise) because i am allready using 1,2gb of the 1,28gb Vram on the card when playing games like BF3. So will i get a performance boost, or will it be the same because they can't share the Vram? My monitor is 1920x1080p.

vRam is not cumulative, as in it does not add up. You are limited to the amount of vRAM on your lowest vRAM card. If you were to add a second one, you'd have a second card to balance the load (and perform more graphical calculations), allowing you to increase your frame rates to a certain degree.

so i assume that i will get an increase in FPS in most games, but not the typical 60% for SLi, am i right?

You could, but it would depend on what the rest of your system is. If its a 775 your second card could bottleneck it slightly, whereas a SB or 1366 build you'd see that average 60% increase.

I heard that using cards with too little ram will impact performance even in sli, but if you say so, i'm going to trust you :)