Let me go with some basic information that you may or may not already know.
First of all, AMD's APU's (your a8-3870k) aren't going to blow anybody away doing anything. There's a reason they're cheap. They do, however, come with good integrated graphics. Not as good as your average discrete gaming card, but good enough for casual gaming. This is their singular advantage: they can do casual computing for cheap with no significant weaknesses. Intel chips can also do casual gaming for very cheap, but they have a significant weakness: their integrated graphics can't handle many games even on very low settings.
I say this because AMD APUs really only make sense for one thing: building a dirt-cheap computer that is nevertheless satisfactory. At no point are they ever going to be good for a "souped-up" or "screaming" rig. You CAN take late-80's Toyota Corrola engine and plug it into a custom Mustang frame - but it just doesn't make sense. That doesn't make a late-80's Corrola engine useless, however., You just need to revamp your expectations of what you're building.
So, a few other factiods:
overclocking doesn't make a huge difference in the a8's performance
1866 speed RAM does help out the a8's integrated graphics performance
motherboards do not generally have a major impact on a CPU's performance unless you're shooting for a high overclock
if you wanted a top-of-the-line gaming rig, you need to start over, and return or just absorb the loss of the a8-3870k
if you just wanted to build your own computer and can find a use for a good casual-computing rig (for a non-gamer, parents, child, actually more computer users than gamers, heh), then you're on a good path
Right now I am struggling trying to decide on a decent FM1 socket board and I am about to start pulling my hair out. All the reviews seem to have some kind of negative about them and frankly I really don't know what I am looking for. I know I want at least 8gigs of 1866 and I want to utilize Crossfire with a 6670 but beyond that I am at a loss.
In-line with the revamped expectations & advice above, I suggest changing your goals here. First of all, you just want a cheap FM1 board. There's a minor difference between a55 & a75 versions, most of which are USB 3.0 capability, which isn't too important. You shouldn't be aiming for much of an overclock (small performance gainst = not worth time or $ for extra cooling for pushing the envelope), so you can pretty much go with the cheapest motherboard available. Exception: using integrated graphics, you need a mobo that has the right monitor connection to avoid having to use an adapter. So pay attention to which mobo comes with HDMI, DVI, whatever you're looking for. Generally the cheaper versions have one of HDMI or DVI & you have to pay $10 more or so to have both.
You probably don't need 8gigs of RAM. 4 would probably be fine. But RAM's not too expensive, so to be honest I'd probably get 8gb myself.
You probably do NOT want to utilize Crossfire with the 6670. Frankly, the 6670 is old and it's not going to add much graphics oomph. If you ask me, you should just stick to real cheap with just the integrated graphics or abandon the integrated graphics altogether and get a real discrete graphics card that blows away integrated+6670 crossfire. Essentially, you already have a good base level. The extra $50-60 for the 6670 will not improve graphics settings all that much, but a $150 or so card would blow away the 6670 Crossfire if graphics settings were important enough to you to spend the money. Maybe if you can find a dirt cheap 6670 used on the marketplace here or something. But mostly, not worth it.
What would I get if I were you, assuming you already have CPU, case, monitor, mouse, & keyboard? (If not, I recommend the Dell u2312hm if you can find it on sale as an excellent quality monitor with no big weaknesses, the Mionix Naos 3200 for the most comfy & gaming-capable right-handed mouse you'll ever own, and a dirt-cheap keyboard.) The following:
Mobo: BIOSTAR A75MG FM1 AMD A75
Why this one? It's $70 on newegg, there's a $15 mail-in-rebate, and it comes with a free 8gb of RAM. As an a75 version of the FM1 chipset, you get USB 3.0 if you ever want it. You probably don't want to shoot for much overclocking on it.
Ram: See above. Now 1866mhz RAM is better for AMD APUs and that mobo comes with 1600mhz RAM... but remember the goal here: satisfactory for dirt cheap. There is no possibility of creating a gaming rig with a lot of power with this CPU. So we shouldn't spend money we don't have to trying. That said, if you wanted to go with another mobo, here's a RAM possiblity: Geil Legara 2x8gb ($38)
PSU: Corsair CX 400 This is a bad week for PSU sales, as there are no good ones. Common good but cheap PSUs to look for are Corsair CX400 (not the CX v2 series), Rosewill green 430w, Antec Earthwatts, Antec Neo Eco 450c, XFX Core 450w, and supposedly Seasonic is coming out with a 'g' series that includes a 360w PSU, something like Seasonic g360, but I haven't seen it available for sale yet. This corsair is quality and costs $40, though sometimes you can find it for $30-$35.
note us.ncix.com offers free shipping, but only for orders over $50, so you may want to pick up a DVD-burner or whatever if you order from there.
HDD: Hard drive actually has one of the larger effects on casual gaming performance nowadays. You generally want a 7200rpm drive of whatever size (faster performance than the slightly quieter 5400rpm drives). SSDs make everything on them load super fast, and may actually be worth it for many people, but we're staying on the cheap side here. You can get 1TB for $70 - 500gb comes at about $55.
DVD-burner: $16 after promo code
(These are all new components. You can, of course, continue to look for used). The components above come to $196 including shipping and will allow you to complete the tower itself, which means you'll get a working computer, sans Windows & peripherals, that's capable of casual gaming (running most any game, but many at low graphics settings) for about $275. To be honest, not something to write home about, but a nice lil' build if you need the computer.