LOL - I found a new sig for you nubius!!! This is perfect for you!
In order for SLI to work properly NVIDIA must make special drivers with profiles for each game to tell the videocards how to work together to display a game.
There are three ways SLI works:
1.) Compatibility Mode - Used in NON-SLI supporting games - Only (1) card renders and there is no performance increase.
2.) AFR - Alternating Frame Rate - Each card works independantly rendering alternating frames displayed continuously.
3.) SFR - Split Frame Rate - Each card renders in two halves then after the two halves are displayed in a single image.
In theroy SLI has the advantage of giving gamers the opportunity to run games / monitors at there highest resolutions and settings so that they will get the highest performance. This is a theroy.
The other reason is that you can buy two less expensive cards and run them in SLI to get the performance of a more expensive card a less of a price. Hence "(2) 6600GT'S in SLI would probably be like a 6800 Ultra. Saving you what? $50 to $75 may-be $100?