Looking for advice regarding PSU and CPU

Im using a Radeon HD 7850 in my ITX build, and I only have a 500watt PSU. You really don't need much more than that. You only ever need above 600watt if you plan to use multiple GPU's or add other components. Since you are using ITX 500watts should work nicely.

Personally, I think FM2 is the wrong choice of platform for what you are trying to do. The Athlon quads are pretty nice entry level CPUs, but for a decent gaming machine they fall quite a way behind. Going FM2 means your upgrade potential is non-existent so you are stuck with a CPU that is fairly weak.

If your budget were lower, and you were trying to match up say a 750K with a more entry level gaming card, perhaps a HD7770 or something, then it might make more sense, though at that level a case as expensive as the Silverstone is (presumably) would take up a lot of the budget.

To get to the stuff you actually asked about -
PSU -
It's understandable you'll get quite a lot of different figures for this. The HD7870XT/LE is a mess of design/marketing, so it's not really a standard benchmark card, so information for it will be slightly hazy at best. Secondly, it can be easy for people to assume that it behaves like a HD7870 (or near enough) when it does not. Despite sitting between the HD7870 and HD7950, it actually uses more power than either. "Boost", "Ghz" and factory OCs make this even more complicated. Aside from that, ITX boards use less power than ATX boards, which will be used in pretty much all benchmarking for "total system power draw". Finally, manufacturers will inflate graphics card figures to cover the huge variety of PSUs on the market.
All of that is a long way of saying that pages like this, while helpful, aren't terribly applicable in this case. A more realistic figure for the card alone is between 200 and 250W (probably nearer the lower end of that bracket) which means it'll still be absolutely fine on a 450W PSU.
Long story short, the 450W Silverstone PSU will suffice, but there is a 550W version in the works if you want a bit more headroom.

CPU
If you are stuck with FM2, then to me it's pretty clear that you want the 750K or 760K. The lower tdp CPUs are fine for basic stuff, but as I mentioned above, I can see the CPU being the weak link in this build so going as far as you can with it makes a lot more sense. I wouldn't have any massive concerns about cooling, the case is fairly well vented and it has a vent specifically for the CPU, though which motherboard you pick will have some influence on which coolers you can use. If you aren't overclocking, then you won't need anything elaborate anyway.
Also, as far as I'm aware all of the CPUs we are discussing (740, 750K, 760K) all have the same memory support - up to 1866Mhz.

I bought the aarock one you mentioned and I love it.