Quote Originally Posted by rfarris View Post
To do SLI do the video cards have to be the same?
They have to be the same.

http://www.geforce.com/hardware/technology/sli/faq#c16 (and the few entries below that as well)

Quote Originally Posted by rfarris View Post
I have a 670 w/4GB. Should I add another 670 and SLI them or should I buy a 770?
A GTX 770 ($500) isn't that much of a boost over a second GTX 670 ($350) for SLI; but with SLI, your mileage is going to vary from game to game.