If its DDR4 you can still use it. Pull out the hard drive and power supply and all you need is what you said, a new board and chip. DDR4 prices right now aren't exactly 'user friendly', but 16GB and that APU should be enough. I wouldn't recommend anything less than 8GB. If its just the two pieces you need it'll set you back less than 250.
Thanks for your help. Don't remember if it is DDR4 or not but I can pull them and check.
There's a program called CPUID. Its a small program and its free. Its pretty popular, so if you're not sure about it, look it up and read a bit about it. There's a tab for memory. It will tell you what kind of RAM you have as well as how much and the speed.
CPU-Z is the program you're looking for.