There's no point in getting the Sandy anymore especially since the price difference isn't even that big. You also won't be able to enjoy the full benefits of your z77 board since it's based on features that came with Ivy.
The 550 Ti is a very weak card, it's not worth the buy. The 6870 or 7770 will outperform it.http://www.anandtech.com/bench/Product/540?vs=541
Here's my revision of your build,http://pcpartpicker.com/p/ny7X
If it's not much of a stretch, I would get a Caviar Blue instead btw.