What does sli do

Solution
SLI and Crossfire enable 2 or more GPUs to work together and thus increases performance greatly. In games that support it, it usually allows you to turn up all the effects in the game.

No, you don't have to have multiple monitors for SLI/Crossfire, but unless running over a 1080p resolution, its a bit of a waste of money. For a single monitor at 1080p you can easily get away with a single GPU. I run 2 1080p displays off of a single Radeon 6870, I can play BF3 on mostly high settings, but since I don't run eyefinity, BF3 plays on only 1 of the monitors. IF I Crossfired my GPU, I could probably crank up the eyecandy and use eyefinity.
SLI and Crossfire enable 2 or more GPUs to work together and thus increases performance greatly. In games that support it, it usually allows you to turn up all the effects in the game.

No, you don't have to have multiple monitors for SLI/Crossfire, but unless running over a 1080p resolution, its a bit of a waste of money. For a single monitor at 1080p you can easily get away with a single GPU. I run 2 1080p displays off of a single Radeon 6870, I can play BF3 on mostly high settings, but since I don't run eyefinity, BF3 plays on only 1 of the monitors. IF I Crossfired my GPU, I could probably crank up the eyecandy and use eyefinity.
 
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