Some misinformation here.
*You can ONLY SLI graphics cards that have the same GPU. You can't pair a GTX770 with a GTX680. No.
You CAN:
1) Use different frequency (not recommended, such as 900MHz vs 1050MHz for each card)
2) Use different VRAM amounts (4GB and 2GB; though you'd default to the lowest and only use half the 4GB amount)
3) Use different model (i.e. Asus GTX770, and MSI GTX770)
*If you can't find another model to SLI then I recommend you SELL it and buy one of the new GTX8xx cards when they are released (GTX860, 870, or 880. The GTX880 would likely be $500 so the GTX860 may be $300 to $350 though it may be even cheaper.)
The GTX880 is estimated so far to be about 50% more powerful than a GTX770. Also, we can get an Asus...