Every time Intel comes out with another iteration of such technology, there are suddenly many questions afterwards about what you can do with a very small SSD--because no perceivable benefit can be noticed. Perhaps detectable in benchmarks (which invariably are run with the minimum possible RAM and a HDD in reviews to show the technology at its best), but not felt.
This happened with Intel Turbo Memory using 2-4GB mini-PCIe cards, Intel Smart Response Technology with 20-64GB SSDs, and now Intel Optane/3D XPoint 16-32GB M.2 NVMe modules
I presume 16GB of RAM would be enough for caching 160-500GB of HDD capacity?
That would depend entirely on how much of that 16GB is normally occupied by programs. Only otherwise unused RAM is available for disk cacheing.
Does AMD have any equivalent software?
Yep, AMD StoreMI version of FuzeDrive works with any X399, 400 or 500-series chipset (presumably you could buy
FuzeDrive for other Ryzen chipsets). Or many SSD manufacturers have their own RAM cacheing software that won't work with HDD, including Samsung Magician's RAPID mode or Sandisk's ReadyCache version of ExpressCache. As with Windows' own cacheing, they cut the usual response time of SSD from ~0.1ms to ~0.0001ms for things in the cache. The Windows implementation works well though, plus has the added benefit of being able to free up the RAM for programs if it is ever needed, and on HDD too.
Note that a 256GB SSD is not sufficient to hold even one game: Gears of War 4 + all DLC is a 350GB install. Just the big update itself was a 248GB download, larger than the formatted capacity of a 256GB SSD. And while AMD StoreMI would be free for your B450 chipset, using a 256GB SSD to cache a 500GB HDD does seem kind of silly. Fortunately it can (unlike Intel) be set as a JBOD where your most frequently used files are automatically moved to the SSD and the whole array is seen as a 750GB C:\ drive, sufficient to hold at least a few games. I probably wouldn't use the RAM cache part though.