gtx 770 or 760 sli

Solution
GTX 760 SLI will have better performance, but you may have microstuttering in some games, it will use more power, in turn generating more heat, and the on-board GDDR5 RAM will not double, it is shared between them to create images. As for pros, it is theoretically double speeds, although never more than 80% at most, and more performance than a single card priced the same.

You should get just a GTX 770. It will still rip through all modern day games at ultra settings, with some AA.
GTX 760 SLI will have better performance, but you may have microstuttering in some games, it will use more power, in turn generating more heat, and the on-board GDDR5 RAM will not double, it is shared between them to create images. As for pros, it is theoretically double speeds, although never more than 80% at most, and more performance than a single card priced the same.

You should get just a GTX 770. It will still rip through all modern day games at ultra settings, with some AA.
 
Solution
Two 760's is a kick-butt rig!
But one 770 has its advantages. Less power, less heat (so less noise).
Less fiddling around with SLI game profiles.
Less PSU requirement - with the two 760's you would need 2 x 2 vidcard power connectors and probably 650W minimum.
One 760 would be good-to-go with maybe 500 W (good-quality though).
Bottom line: I would be happy with a single GTX 760! I'm going to get one, too.
But a 770 should be killer. As long as you're not going super high-res, ie. over 1920 x 1080, you won't need more.