That will be a sweet little gaming rig Wries, and to an extent a far cry from a power efficient HTPC. You could always use a raiser card and mod a bigger GPU in it for more horsepower. Will you be using local storage only? 4TB seem a bit excessive; I'd rather run a NAS myself in that case.
My plan was to get a Trinity APU and a mITX motherboard together with a picoPSU in a Streacom F1C case. I have sheleved the idea for now though.