First Ever Build, Sub-$600: AMD or Intel for floating point; CFD/games

I'm looking to build a mid-level desktop this summer for ~$550. Being a home computer, my main usage will be basic productivity and internet use, but I am looking to upgrade my hardware for two reasons:
1. CFD simulations
2. Low level nostalgia/legacy gaming (current most demanding games are Civ 5 and Dragon Age: Origins, but I might pick up Skyrim and FIFA xx)

So my priorities for the system are
1. Good processor
2. Cheap
3. Adequate graphics capability

I have access to beefy workstations for CFD at my university; I'm not looking to duplicate that capability at home, but rather to give myself the flexibility to prep and do preliminary runs from home. My research tells me that CFD runs are heavy on floating point calculations and are severely bottlenecked by memory bandwidth. I initially looked into quad channel memory CPU options, but couldn't find solutions to fit even an expanded $700 budget (Core i7 extreme or dual-CPU server builds were $$$$$). I decided to look for 4+ core units I could afford.

The three processors I've found are: A10-xxxx APU, FX-6300, and Core i5-4590. I have looked at a bunch of CPU comparisons on Tom's, so I know the Intel is head and shoulders above the AMDs in a general sense, but any pointers about floating point performance? Which benchmarks test floating point? Also, from a cost efficiency perspective, even after looking at APU reviews, I couldn't figure out why the Kaveri A10-7850k should be 50% more than the Richland A10-6800k.

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Below are the builds I have come up with. I welcome input on any component, but especially the case, PSU, and mobo, about which I know almost nothing.

The FX:
PCPartPicker part list / Price breakdown by merchant

CPU: AMD FX-6300 3.5GHz 6-Core Processor ($109.99 @ Amazon)
Motherboard: MSI 970A-G43 ATX AM3+ Motherboard ($59.99 @ Newegg)
Video Card: Sapphire Radeon R7 260X 2GB Video Card ($99.99 @ Newegg)

Total (including the rest of the rig, see below): $537.84
Generated by PCPartPicker 2014-06-13 21:29 EDT-0400

The APU
PCPartPicker part list / Price breakdown by merchant

CPU: AMD A10-6800K 4.1GHz Quad-Core Processor ($119.97 @ Amazon)
Motherboard: ASRock FM2A75M Pro4+ Micro ATX FM2+ Motherboard ($59.49 @ Newegg)

Total (including the rest of the rig, see below): $447.33
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-06-13 22:45 EDT-0400


And the Intel:
PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i5-4590 3.3GHz Quad-Core Processor ($188.00 @ Fry's)
Motherboard: MSI Z87-G41 PC Mate ATX LGA1150 Motherboard ($66.50 @ Newegg)
Video Card: Sapphire Radeon R7 250 2GB Video Card ($71.98 @ Newegg)

Total (including the rest of the rig, see below): $594.35
Generated by PCPartPicker 2014-06-13 21:31 EDT-0400

Rest of the Rig:

Memory: G.Skill Ripjaws Z Series 8GB (2 x 4GB) DDR3-2400 Memory ($74.99 @ Newegg)
Storage: Samsung 840 EVO 120GB 2.5" Solid State Drive ($84.98 @ OutletPC)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($52.92 @ Amazon)
Case: Rosewill Galaxy-01 ATX Mid Tower Case ($29.99 @ Newegg)
Power Supply: Corsair Builder 430W 80+ Bronze Certified ATX Power Supply ($24.99 @ Newegg)


And some final motherboard questions:
Motherboard futureproofing - any features I should look for?
For the AMD builds, I might play with overclocking - are those mobos ok?
According to (this article on Tom's, memory performance peaks at 2133 Mhz - should/can I underclock the 2400 memory on these systems? Sticks of 2133 were the same price as 2400.

Thanks so much to anyone who made it through this marathon post!