Well lets just say, sandy bridge = better for overclocking but overall the ivy bridge is newer and better and why not get it? yes there is a difference the i5 has 4 cores and the i3 has 2. The i5 3570k is The best gaming cpu's you can get
You can swap it with asrock z77 extreme 4, or for cheaper just grab a asrock z77 pro3...
If you do want to spend in that high area then Swap out the gpu in the above build and replace it with this http://www.newegg.com/Product/Product.aspx?Item=N82E168...,
And now with these changes and in the future you can easily swich your cpu to a 3570k without upgrading the mobo, you will have a kick ass system