AMD Smart Access Memory support already available in beta form for select X470 and B450 motherboards, full support coming in January 2021
Unofficial support for the new Smart Access Memory feature (and the Ryzen 5000 CPUs, implicitly) is now being rolled out in beta form on select X470 and B450 motherboards. Thankfully, AMD did not limit this feature to the new more expensive X500 motherboards. Still, SAM will not work on the Ryzen 3000 or older Zen / Zen+ versions. In order to benefit from the SAM function that allows the CPU to get full access to the GPU memory, owners of the X470 and B450 mobos need to buy one of the new Ryzen 5000 CPUs, as well. Furthermore, SAM only works in conjunction with the RX 6000 GPUs for now, but if you own an Ampere-based card, you will be able to get something similar to SAM as Nvidia and Intel will introduce support for the Resizable BAR function in a few months.
The motherboards that are currently offering beta BIOS versions supporting the new Ryzen 5000 SAM feature include all the B450 models from ASRock and all the X470 / B450 models from Gigabyte. MSI and Asus also have unofficial BIOS versions for their X470 and B450 models, but these are only available for testers and cannot be downloaded directly from their support sites.
If you really know what you are doing, you can try the unofficial versions, but keep in mind that these are unofficial BIOS updates that are not covered by warranties. If you do not mind waiting for a bit longer, you could get the official AGESA V2 22.214.171.124 as early as January 2021.