SSD all the way. If you already planned on getting an SSD anyway, then yeah I'd say cpu would be a good buy.
The only real issue is that if you upgrade your cpu it's likely going to result in need for a different motherboard. Unless you really want to be stuck on an APU, which I can't really suggest with a rig thats packing a 380 in it.
I go with @Raate, no need for an APU when using an R9 380. Also… I'm not crazy about your choice for the motherboard… I'm not really familiar with the chipsets for AMD CPUs, but your motherboard looks quite entry level and featuring quite few connectivity options. It doesn't look like it is future proof.
CPU changes were discussed in your original topic putting that build together, the only other CPU I'd bother with at that price point'd be the pentium G3258, which has better single core performance but is a dual core... the former is why I chose to go with that athlon, games that can utilise multiple threads'll run better on it, and they're presumably going to be becoming more abundant in the future. The other benefit of the pentium is that it's socket 1150 so you've got a better upgrade path (could jump all the way up to a 4790K if you wanted on the same motherboard), buuuuttt by the time you'd be upgrading socket 1150'll be outdated by new socket types. Skylake's here with the new 1151 socket type, so if you wanted to upgrade to a processor with that socket type later down the road (as you probably will, socket 1150 chips'll probably be harder to find by then and not necessarily even cheaper), you'll have to get a new motherboard anyway.
I'd say spend it on an SSD. If you've money left after that, upgrade your PSU to something 80+ Bronze and/or maybe look at some other motherboards.
You can't get any more "future proof" with that motherboard without changing the CPU - it's FM2+ and that athlon's pretty much the best CPU with that socket type. Better OC performance, per se, could be gained.
No need to worry about the case, ATX, microATX and miniITX are the standard form factors. An ATX case will fit all the other ones as it is the biggest. Motherboards for AMD CPUs sometimes use the DTX form factor, and I'm not sure if you can fit one in to an ATX case, but this won't bored you.
MicroATX is a good choice for someone not intending to use more than (usually) 3 expansion slots, which would be the majority of home users (me included). The downside is that it may happen that there's usually less competition for this form factor which will drag prices up, but this only usually applies to high end motherboards.
Now when I think about good AMD CPUs on the cheap side the, the FX series comes to mind. And this would be my pick. As @SpaceCat just mentioned, this requires a motherboard change for the AM3+ socket. A move which I believe is much recommended!
The AMD A10-7870K you currently have listed sells for around 140 USD. A good, recommendable, FX CPU would be the FX-4350 around 90 USD. The FX-8370 is an eight-core that could be get for under 200 USD but it would not represent a considerable increase in performance, specially for gaming. So I'd strongly recommend the FX-4350, which can also be over clocked should you get a motherboard capable of doing so.
Assuming you are on a budget, but having in mind you'd be saving 50 USD on the CPU change some interesting motherboards would be: 1) ASUS M5A78L-M/USB3 (USB 3.0, microATX, 4 RAM slots, plenty connectivity) - around 65 USD; 2) ASUS M5A97 EVO R2.0 (an even better board, ATX) - around 80 USD; 3) ASUS M5A99X EVO (my pick) - around 100 USD; 4) MSI 970 GAMING (my pick also) - around 100 USD.
I have no experience with over clocking AMD CPUs so if someone could chime in it would be great.
Aye, the 860K beats it and is cheaper though, so the FX-4350'd be a downgrade in performance. Not, however, in upgrades. Leaves you a wider range of better CPUs to get without a motherboard change... whether it'd be worth it sorta depends how long it'll be until upgrades are probably going to happen I suppose.
@SpaceCat, why do you reckon the 860K is faster than the 4350? I know it's a more recent CPU, but it's a worst performer in every single parameter you can compare.
I honestly think the 860K was launched just to compete against the G3258, as most gamers on a budget were changing to Intel. The FM2+ socket is being deprecated by AMD, this is the only way the 860K makes sense to me: as competition for the G3258. And a rather lame attempt, as the G3258 is a far better CPU and the one I chose for a recent cheap build.
The one thing I would say with the AMD upgrade path is the logical upgrade next year would probably be a Zen CPU opposed to a higher end FX part and the Zens are near certainly going to be on a new socket (rumoured to be FM3), so its probably going to be a new motherboard too.
The problem with upgrading on AM3+ is that your per core performance isn't going to change much, so for gaming it's not going to help most games.
Plus FM2+ has USB 3.0 on the cheap, not sure about AM3+ since it doesn't natively support it, so the best thing to do most likely would be to go on the cheaper end, and wait for zen/skylake 2.0 for next year.
You can get USB 3.0 on motherboards with both of those sockets, and for a similar price. I'd go for the AM3+ gambling that AMD would in the future release a faster FX quad core.