Thoughts on my first build much appreciated.

I would say jump for GTX 960 and an FX 8350 which would put you at just slightly below your budget ceiling of $850. However, this build would work as is if you wanted it to. Do you plan on overclocking the fx 6300? If so then the idea of getting the 8350 becomes even less needed.

I would only advise going PileDriver on AM3+ if you intend to overclock for the sake of overclocking and are intrigued by the novelty factor of the AMD option.

PileDriver CPU's rarely offer a better value for gaming builds. Core count and clock speeds are not measures of execution resources or performance unless comparing equivalent architectures.

To help give you some perspective, a Haswell core, is "larger" in terms of execution resources, than piledriver module. (A piledriver module, is 2 cores). Gaming workloads scale better into core performance than core count, so piledriver has a distinct disadvantage for gaming workloads right off the bat. Any gaming build that leverages piledriver represents a significant compromise in one of the most important performance attributes of a gaming computer; single threaded performance. General rule of thumb is that in compute bound conditions in games, haswell offers 25-75% higher minimum FPS for a given implementation cost.

Real-time view-port performance in professional applications for CAD/modeling/design etc draws on a lot of the same API's and technologies that a game engine leverages, and is inherently also heavily bound to CPU performance in similar ways. Ironically, while you will find lots of people using PileDriver for gaming builds, you won't find hardly anyone using it for professional viewports. The professional world has no interest in compromising for novelties sake, but the non-professional world seems to be heavily influenced by an immeasurable novelty factor that has no objective basis.

As long as you understand the compromise and the effect that it will have, there is nothing wrong with building a gaming machine with an AMD CPU. There are plenty of games that run just fine on piledriver, but I would say that if you play any of the following games you might want to consider Haswell more seriously: WoW, WoT, RoboCraft, Starcraft 2, Supreme Commander 2, Arma series and Arma mods like dayZ, Modded flight simulator programs, (just to name a few popular games that are notoriously CPU intensive and highly dependent on single threaded performance, there are more)

When using AMD CPU's, you'll probably want to use Nvidia GPU's to help alleviate some of the compute bottleneck. Nvidia's DX11 implementation is better optimized for intercore parallelism, and as such, raises the performance floor tied to single threaded performance in compute intensive DX11 games by ~25% in congested conditions. Using an Nvidia GPU and overclocking the CPU can actually get FPS minimums on an AMD CPU build up to the same class as an i5-4460 paired with an AMD GPU in many DX11 games.

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So, assuming you're set on going AMD in a mATX case for <$850

CPU: AMD FX-8310 3.4GHz 8-Core Processor ($114.99 @ TigerDirect)
CPU Cooler: RAIJINTEK AIDOS 48.6 CFM Sleeve Bearing CPU Cooler ($23.17 @ Amazon)
Motherboard: Gigabyte GA-78LMT-USB3 Micro ATX AM3+ Motherboard ($59.99 @ Amazon)
Memory: Crucial Ballistix Sport 8GB (2 x 4GB) DDR3-1600 Memory ($61.99 @ Adorama)
Storage: OCZ ARC 100 240GB 2.5" Solid State Drive ($89.99 @ Amazon)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($52.20 @ Directron)
Video Card: Zotac GeForce GTX 960 2GB Video Card ($199.46 @ Directron)
Case: Cooler Master N200 MicroATX Mid Tower Case ($44.98 @ Newegg)
Power Supply: Rosewill Capstone 550W 80+ Gold Certified ATX Power Supply ($69.99 @ Amazon)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($17.92 @ Amazon)
Operating System: Microsoft Windows 8.1 - 64-bit (OEM) (64-bit) ($92.00 @ B&H)
Total: $826.68

In order to really do the AMD build "right" you'd want to make the following changes:

CPU: AMD FX-8310 3.4GHz 8-Core Processor ($114.99 @ TigerDirect)

CPU Cooler: RAIJINTEK THEMIS 65.7 CFM Sleeve Bearing CPU Cooler ($20.99 @ NCIX US)
Motherboard: ASRock 970 Performance ATX AM3+ Motherboard ($99.98 @ Newegg)
Memory: Crucial Ballistix Sport 8GB (2 x 4GB) DDR3-1600 Memory ($61.99 @ Adorama)
Storage: OCZ ARC 100 240GB 2.5" Solid State Drive ($89.99 @ Amazon)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($52.20 @ Directron)
Video Card: Zotac GeForce GTX 960 2GB Video Card ($199.46 @ Directron)
Case: Corsair 200R ATX Mid Tower Case ($49.99 @ Amazon)
Power Supply: Rosewill Capstone 750W 80+ Gold Certified ATX Power Supply ($84.99 @ Newegg)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($17.92 @ Amazon)
Operating System: Microsoft Windows 8.1 - 64-bit (OEM) (64-bit) ($92.00 @ B&H)
Total: $890 shipped

Note the motherboard, HSF, case, and PSU all have the headroom to go nuts on overclocking.

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If you don't want to have to mess with performance tuning and tinkering with settings to attempt to alleviate compute performance problems (common on piledriver gaming rigs), do the following:

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i5-4460 3.2GHz Quad-Core Processor ($179.59 @ SuperBiiz)
Motherboard: ASRock H97M PRO4 Micro ATX LGA1150 Motherboard ($74.98 @ Newegg)
Memory: Crucial Ballistix Sport 8GB (2 x 4GB) DDR3-1600 Memory ($61.99 @ Adorama)
Storage: OCZ ARC 100 240GB 2.5" Solid State Drive ($89.99 @ Amazon)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($52.20 @ Directron)
Video Card: HIS Radeon R9 280 3GB IceQ OC Video Card ($182.98 @ Newegg)
Case: Cooler Master N200 MicroATX Mid Tower Case ($44.98 @ Newegg)
Power Supply: Rosewill Capstone 450W 80+ Gold Certified ATX Power Supply ($59.99 @ Amazon)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($17.92 @ Amazon)
Operating System: Microsoft Windows 8.1 - 64-bit (OEM) (64-bit) ($92.00 @ B&H)
Total: $856.62
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-02-24 05:23 EST-0500

Well it's okay if overclocking doesn't really sound like your thing. After doing some research here's a build I'd recommend:
http://choosemypc.net/build/?budget=850&oc=false&options=,removessd,os
It comes with a gtx 770 which is faster than a R9 280 and it has an i5 as the processor which is good for gaming. This build should max most games out at 1080p.