The question is that do you really needs all 16Gb of RAM now or is it better to only buy 8Gb first and invest the rest of the money on improving GPU, CPU or getting an SSD. Unless you are running some memory heavy program then upgrading to an 560Ti would bring more performance to your system then additional 8Gb of ram. Even with resource intensive program, its going to be bottle necked by the i3 and weaker GPU.
As the z77 board will be available in stores very soon, it would be wise to get on of those instead of a z68.
Sandybrige and Ivybridge uses dual channel memory controller, it would be best to have memory sticks in multiple of 2. Since your originally was going to get 2 of the 8Gb then it doesn't really matter, might be a problem if you only get one.