OP: There's no reason to go above 16GB unless you have a specific workload that demands it - heavy (4k or above) video editing, CAD, 3D modelling, VMs, etc. Other than that, 8GB is passable for most use, and 16GB is usually plenty (unless you're one of those people who leaves 100 Chrome tabs open at all times and then complains of it being a memory hog).
Still, buying new RAM for a system that old is only worth it if the RAM is very cheap. If not, you'd be better off buying a brand-new system, even if the total cost will be higher - even a low-end modern CPU will be significantly faster than your Phenom. Even a Ryzen 5 3400G is nearly double the speed of your CPU according to Passmark, with a modern 6-core like the Ryzen 5 3600 being almost 4x the performance. In short: investing noticeable money for things that can't carry over to a newer PC for a system like that is likely not worth it, and you're likely to get far more for your money if you save up for a full system upgrade.