The
FX-8350 has one integrated dual-channel memory controller which natively supports up to DDR3-1866.
The quality and capabilities of the iMC vary from part to part, some CPUs will be able to support faster memory, a few will even be epic, others will not. AMD rates this CPU for DDR3-1866 (if not then it's a defect and can be replaced under warranty), but there's never any guarantee about thresholds when overclocking.
Some online searches show that most overclockers report using DDR3-2133 or DDR3-2400 offers best results. The CVFZ fares a little better than most of its competitors, but even so it achieves DDR3-2400+ only when paired with an "above-average" processor part.
So don't get anything slower than DDR3-1866. And don't waste your money getting anything faster than DDR3-2400 (fastest OC officially supported/expected on your mobo). Because of the way the dual-channel memory controller is hardwired you probably won't see any real difference between a 4x4GB kit and a 2x8GB kit, nor much real difference between Single-Sided and Double-Sided DIMMs.
This G.Skill 2x8GB DDR3-2133-9-11-11 1.6V kit seems like a good deal at £58, better than the DDR3-1866 kits which cost about £55, and the next step up is DDR3-2400 which starts around £70. The G.Skill, HyperX, and Crucial DDR3 products look reasonably priced; the Corsair, Mushkin, and GeIL DDR3 products look overpriced; I'm not really familiar with other DDR3 brands, and I'm not sure if those prices from one vendor are really representative of prices in your area. You might choose to pay a little more if you're particular about brand or colour/style, etc.
You might require a little extra cooling for your RAM. I personally think adding/upgrading fans to increase chassis airflow is generally a better approach than spot-cooling with RAM Cooler accessories, but opinions and styles and results do differ.
"All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others." - Douglas Adams
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