Seeking Advice on New Build

Roughly? you could have something much better

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i5-7600K 3.8GHz Quad-Core Processor ($232.99 @ SuperBiiz)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($27.88 @ OutletPC)
Motherboard: Asus PRIME Z270-K ATX LGA1151 Motherboard ($142.99 @ SuperBiiz)
Memory: GeIL EVO POTENZA 16GB (2 x 8GB) DDR4-2133 Memory ($82.99 @ Newegg)
Storage: Sandisk Ultra II 480GB 2.5" Solid State Drive ($132.99 @ Best Buy)
Video Card: MSI Radeon RX 480 8GB GAMING X Video Card ($253.98 @ Newegg)
Operating System: Microsoft Windows 10 Home OEM 64-bit ($88.58 @ OutletPC)
Total: $962.40
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-02-03 23:25 EST-0500

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i7-7700 3.6GHz Quad-Core Processor ($309.33 @ OutletPC)
CPU Cooler: CRYORIG H7 49.0 CFM CPU Cooler ($34.88 @ OutletPC)
Motherboard: MSI B250M GAMING PRO Micro ATX LGA1151 Motherboard ($84.99 @ B&H)
Memory: Patriot Viper Elite 16GB (2 x 8GB) DDR4-2133 Memory ($81.99 @ Newegg)
Storage: Sandisk Ultra II 480GB 2.5" Solid State Drive ($132.99 @ Best Buy)
Video Card: Gigabyte Radeon RX 480 8GB G1 Gaming Video Card ($234.99 @ Jet)
Operating System: Microsoft Windows 10 Home OEM 64-bit ($92.99 @ B&H)
Total: $972.16
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-02-03 23:35 EST-0500

You have options. This is 8 cores that'll blow the doors off that FX and will be good for at least 5 more years. The FX has already had its 5 years and is already slated for replacement by Ryzen.

don't worry, intel products are good. I have gone from the AMD to INTEL and it's not a mistake, at this point anyways.

here, look at something like this.

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i5-7500 3.4GHz Quad-Core Processor ($197.99 @ OutletPC)
CPU Cooler: Rosewill RCX-Z775-LP 33.5 CFM Sleeve Bearing CPU Cooler ($12.99 @ Newegg)
Motherboard: Asus PRIME B250M-A Micro ATX LGA1151 Motherboard ($84.99 @ SuperBiiz)
Memory: GeIL EVO POTENZA 16GB (2 x 8GB) DDR4-2133 Memory ($82.99 @ Newegg)
Storage: OCZ TRION 150 480GB 2.5" Solid State Drive ($129.88 @ OutletPC)
Video Card: MSI Radeon RX 480 8GB GAMING X Video Card ($253.98 @ Newegg)
Operating System: Microsoft Windows 10 Home OEM 64-bit ($88.58 @ OutletPC)
Total: $851.40
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-02-03 23:52 EST-0500

also take a look at newer amd zen(ryzen cpu).

Amd used to be a good budget option, back a few years, always came in second, but it was not a big margin between the higher tier Intel and the high tier FX. That has since changed. Intel has upped its game. Now even the lowly i3-6100, a dual core +HT cpu can handily beat the fx Cpus in a good majority of games. Nowadays the FX doesn't even compete against the i5's in almost any game. And in the games where the 8 threads of the FX are actually of any use, the i7 pretty much destroys it.

Same build, with i5 instead

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i5-7500 3.4GHz Quad-Core Processor ($197.99 @ OutletPC)
CPU Cooler: CRYORIG H7 49.0 CFM CPU Cooler ($34.88 @ OutletPC)
Motherboard: MSI B250M GAMING PRO Micro ATX LGA1151 Motherboard ($84.99 @ B&H)
Memory: Patriot Viper Elite 16GB (2 x 8GB) DDR4-2133 Memory ($81.99 @ Newegg)
Storage: Sandisk Ultra II 480GB 2.5" Solid State Drive ($132.99 @ Best Buy)
Video Card: Gigabyte Radeon RX 480 8GB G1 Gaming Video Card ($234.99 @ Jet)
Operating System: Microsoft Windows 10 Home OEM 64-bit ($92.99 @ B&H)
Total: $860.82
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-02-04 00:46 EST-0500

both of the CPU in question are great. The i7 would be of service to you for longer than the i5. Since it's got more cores, and overall a stronger CPU. But we're talking about... i5 - no problems with games for 3+ years, i7 no problems with games for maybe around upto 5 years. But this is all a rough estimate, since we can't really predict what's gonna happen that far down the line.

If you're working ona budget, getting your hands on an i5 is good enough. But you are asking about 7600k, this requires a Z motherboard, which is more expensive as well as a CPU cooler (these items needed for overclocking, as well as the 7600k does not come with a stock cooler). i5-7500 comes with a stock CPU cooler, so if you're short on cash, you can just use it and upgrade down the line at some point, as well as it does not need a fancy motherboard, so overall, save.

as for the GPU, they're just about the best value/performance, that's why those are being suggested, but I think the visual aspect goes to MSI.

You'll be fine with the PSU you got, that's why no one changed it

The use of an i7 has long been an issue. For all intents and purposes an i7 is nothing more than an i5 with hyperthreading enabled. In the past, the dual core cpus were mainstream, the quads enthusiast class. This mentioned that all games were designed to run on 2 threads at most, but could possibly make use of 4. There are lots of dual core pc's out there still, and thats a market game designers for the most part don't want to skip. There are some games that have taken a chance, GTA:V, and said straight out, needs a quad core cpu. Ballsy. And that's going to be the trend as games literally have so much data that needs processing, and is growing larger, that quads will become the minimum for games in the future. So will GTA:V run on a quad? Absolutely. However, it's still massive. Now is when that previous trend comes back around, the c2d - c2q optimising. You have the i7. It's got 4 cores that due to hyperthreading will shove through 2x threads per core. Effectively 8 cores. Will GTA:V take advantage of those 8 threads? Absolutely. As do all of the games like it, Starwars Battlefront, Battlefield 1,4 Watchdogs 1,2 World of WarCraft etc. Better performance all around, especially after the introduction of mods.

So, do you need an i7? No. A quad is good enough. Can it help? In some games, yes, in some games no. Is the price justifiable? That's for you to decide.

Gigabyte makes good performance gpus, top line out of the box, as does MSI. The MSI gpus tend to run a little cooler, a little quieter (imho) because of their slight advantage of a better cooling system. The heatsinks are bigger, allows for more OC headroom. But if you don't OC the gpu much, if any, there's no real difference other than price. Both are equitable out of the box.

Corsair HX was a fantastic psu, in its day. Still is a workhorse. But it's a group regulated design that can have issues with Haswell or later designed cpus. Namely when the pc goes into sleep mode, it won't wake up. There are workarounds for that, so if that's good for you, it's a good psu and depending on its actual age, could still be a viable psu. For a little while longer. Give you time to save up for a new one. At 650w it's good for any of the above builds.

If you already have an SSD, you won't need more really, I have a 120 and it's fine for OS and extras.

You might want to look into your case size, the hyper212 is cheap but kinda tall and won't fit in a lot of smaller cases, the cryorig H7 would be a wiser choice.

Choice of options can depend on if you want to OC or not, having the 8 threads of the i7 would be the trade off vrs 4threads and OC. Also, with skylake /kaby-lake cpus, the stock cooler is quite good, so you may not even need to spend extra for an add on cooler, only for k models is one necessary since there is no cooler included.

MATX mid-mini towers fit mATX or quite often mITX board (smaller).
ATX mid-mini towers fit ATX, mATX and sometimes mITX.
Full towers basically fit E-ATX, ATX, mATX mobo's.

You don't need a full tower to fit a standard. ATX mobo which carry 2-3 pcie x16 slots, a standard ATX mid tower is built exactly for that.

Price point local US is @$160-180 for a 780t, which is a huge case and double the space/price for any needs you have. Any ATX mid tower will easily fit all your components including sli gpus for half the cost and a greater variety of styles, colors, availability, ease of customization, greater cooling potential (anything beyond 4x fans is almost pointless- a 200mm or 2x 140mm intake, a 200mm or 2x140mm exhaust). Full towers like your storm trooper or the 780t have only 2 purposes, e-atx builds or full custom loop liquid cooling. Anything less is overpriced, wasted space.

Well the gimmick with mATX is that while intrinsically they are just a 3" shorter ATX mobo, what really happens is that that 3" of missing board contains usually the 2nd/3rd pcie slot, all the HD audio, Sata/USB connections etc. So this is where mATX suffer compared to ATX. Generally ATX gets 6-8 Sata headers, 4-5 fan headers, 4 USB2 headers etc. MATX get lucky to have 4 Sata, 3 fan and 2 USB 2 headers. It's not just the price that's chopped along with 3" of board, its many of the things many ppl could use. If you don't need the headers, it's fine, but I have an SSD, hdd, 2x optical, and a card reader on Sata headers. So an mATX would make no sense to me that way. Many mATX have just 1 pcie x16, for a single gpu, that's fine, but that means absolutely no chance of sli or crossfire possible. Many mATX have just 2x ram slots, most only use 2x dimms for a dual channel, but there are some who prefer to use 4x because they bought upgrade ram to make 16gb cheaper than buying 2x brand new 8Gb sticks. MATX has limitations other than its price point, unless you get the really expensive boards, you'll not get the variety of options available to a standard ATX. Many mATX have just 2x fan headers, a cpu and a sys. Very limited on options there without buying an extra fan controller.

For a plain system running minimal hardware, mATX is fine, it has enough. But limited expansion, limited options isn't always a good thing for the few $ price difference.