I disagree with skit, you can leave the 16 gigs of ram the only thing i would change is the psu. The cx are not meant for gaming i would personally recommend the evga g2 series a 500 watt is more than you need but leave some room for upgrading. Whether or not you need a 2tb hdd is up to you. I personally am about to buy another ssd since after a while my 1tb ssd is filling up so i can understand your need for more storage.
The reason i chose a psu of 600w is because i used an online tool to calculate how much power i would need and it came close to 500. Thought it would be safer to have a cushion. Also, what OS should i use? Windows 10?
Below are what parts are bad in your build
Processor (Good )
^Quad cores are favorable for today's latest titles
Cooler (Good )
Price to performance is amazing
Motherboard (Terrible )
Asrock makes very poor quality motherboards
Ram ( Too much)
Only 8Gb is needed
Storage ( Too much)
1TB + 240SSD is a good pair
Video card ( Okay )
Nvidia failed on their latest drivers . 3.5 Usable .5 Slow
Case (Great!)
Power supply (Poor )
Corsair makes very poor quality power supplies and should be avoided
My build : http://pcpartpicker.com/p/37LLvK
The reason i chose a psu of 600w is because i used an online tool to calculate how much power i would need and it came close to 500. Thought it would be safer to have a cushion. Also, what OS should i use? Windows 10?
Personal preference really. I still use windows 7 but i have no experience with windows 10 for gaming. Windows 7 works great with almost all modern games i have heard of the occasional problem with windows 10 and some games from a couple years back. Honestly if your going to be playing games from the last couple years onwards your best bet will probably with windows 10. I think microsoft is releasing some games onto the windows 10 store that are exclusive there. Just make sure when you install it to uncheck all of the shady security stuff they try to install. Like monitoring what you type and etc.
The main difference will be in the number of dimm slots (how much ram you can hold)
Number of pcie 3.0 lanes (how many video cards)
some may support sli and not crossfire or vice versa and some support both.
Some may have 2 pcie 3.0 slots but 1 takes 16 pcie lanes and the other 1 only takes 4. The differences can get complicated but for the most part its simple. Without getting super complicated your cpu has X amount of pcie lanes. For sli you need each card to be in x8 mode. Where as in crossfire they can be in x4 mode. The most pcie lanes a pcie slot can handle as of now is 16.