If you don't sli then the game will only use 1 gpu, not both, so would have less performance. Video rendering can still use both gpus not in sli depending on the software. You can have all of the monitors hooked up but only 4 displaying at once in sli. You can always just switch which monitors are displaying or just disable sli when you don't need it and want all the other monitors on.
An i7 and 16gb is fine. You could probably be fine with 8gb. Although you should get 2 sticks so you have room to upgrade if needed. Be aware you need w7 pro, ult or any w8 for more than 16gb ram.