The chipsets: X370 vs. B350 vs. A320
So far, there are three chipsets available for socket AM4: X370, B350 and A320. In terms of positioning they are comparable to Intel's Z270, B250 and H110. Also, AMD has not developed the chipsets themselves, instead they outsourced it to ASUS-daughter ASMedia, which is known for their among other things their USB 3.1 controller chips. They're produced by TSMS on the relatively old 55 nm process.
X370 is the most high-end chipset and it's the only chipset that allows you to split the sixteen PCI-Express 3.0 lanes into two times eight lanes. This means that you'll only be able to combine two graphics cards in CrossFire or SLI with an X370 motherboard - in theory CrossFire is also possible on the B350 chipset via an x4 slot from the chipset, however we wouldn't recommend this.
B350 lacks this feature and furthermore it comes with less USB 3.0 and SATA ports, however you do still get the option to overclock. You're limited to overclocking via the multiplier, which is not that much of an issue in our opinion - this is by far the most common overclocking method under 'normal' users. Those who want to squeeze all the performance out of their processor with the base clock, will have to opt for an X370 motheboard with a so-called baseclock generator.
A320 is the entry-level chipset and it's mainly meant to be combined with an APU. The number of USB 3.1 ports is halved - although AMD still has an advantage compared to Intel just because they offer USB 3.1 through the chipset - and overclocking is no longer possible. In practice the power supply of many A320 motherboards is not sufficient for a Ryzen processor, so we'd always go for at least a B350 motherboard if we were building a Ryzen machine.
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At the time of Ryzen's launch AMD announced the X300 chipset, which would be released at a later time. The chipset was meant for Mini-ITX motherboards, however unfortunately we've not heard anything since then. The series of special chipsets for ITX boards seems to be cancelled and ITX boards based on B350 and X370 are available right now, however these were released quite late on the market due to the uncertainty about the X300 chipset.