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So will a A10 5700 be powerful enough for 3D playback?

Any advice on the build is appreciated

AMD Radeon HD 7660D iGPU should be enough for 3d content. Here is the spec "AMD A-Series Processor-in-a-Box" and the 3dmark is a lot higher than the latest Intel HD4000 too "IGP performance".......

Since your case support Micro-ATX mobo, why not go for the Micro-ATX mobo? It'll give your more room to install memory sticks and CPU cooler, and it'll give you more options for future upgrades too- ASRock FM2A85X Extreme4-M....