You cant buy windows 7 anymore, unless you find a box copy somewhere. Win 8.1 runs a lot smoother and faster than 7. If you want to get version 10 when it comes out they are offering free upgrade to it from any current version you have.
Also that PC Parts Picker is garbage. 80% of the stuff you picked out is overpriced. I can find it cheaper of sites like Newegg. If anything, find a PC retail store. I have Fry's Electronics and they price match everything. I recently purchased a EVGA GTX 970 FTW from frys for $350. They had it priced at $390 and so they price matched it with Newegg's price.
I would also agree with some of what Thach has here but go with the i7. Its coming up on that time of the year when Intel releases their new shit(June or July). What this means is their older stuff will drop in price soon. Invest the most in your CPU, GPU, and RAM(16GB Max). SSD's are great but if you on a budget wait on that stuff. The CPU heatsink usually comes with the CPU but its not a great one(better than not having one at all, just don't overclock). Cases can be found rather inexpensive and well under the $100 line. Just look for a ATX MID tower. The Case, Monitor, Mouse, Keyboard... just shop around for. Its what your going to be looking at 98% of the time.
But to sum it all up, you can always upgrade over time to get the really good stuff.
Intel i7-4770 Quad-core 3.4GHz so you get the multi-threading that i5's don't have = $300
G.Skill Ripjaws X Series 16GB DDR3 1600 (PC3 12800) = $130
MSI z97-Gaming 5 or ASUS z97-A LGA 1150 Motherboard = both $150
GeForce 960 or 970 GTX is nice and won't be all that expensive when you're planning on buying = anywhere from $200-$400 probably
750-800w Power Supply = $95
250GB SSD = $100
2TB HDD = $100
CPU Heatsink + Fan will cost ~$40 for air cool
Case: Whatever you can afford, but the Fractal Design R5 is an amazing case = $110
Monitor: Something around 1ms response time and the faster the refresh rate, the better
Mouse: Whatever you want
Keboard: Whatever you want
That's a quick base list that you can use that will give you a pretty good gaming computer and have it last, plus you can mix and match different components with this to get it down to a cheaper price point. Finding things on sale and buying them is always a good idea too.
Edit: Note that you could always move down to an Intel i5 as games rarely, if ever, take advantage of more than 2 threads, so unless you are planning on doing video editing and rendering, things of that nature, than an i5 may be more in line with what you are looking for. And it'll be cheaper.
