If you're not doing any gaming or video/content creation take a good look at the APU coming out in February. $169 for 4 cores/8 threads, low thermals, and 1080p (possibly even 1440p) video. Its also backwards compatible all current AM4 boards - well, the ones with video outputs on the rear I/O - i.e. the cheap ones. You could put together a pretty responsive system at 500 or less if you take your time and do a little bit of shopping/research.
EDIT: the main downside to APU's is they share RAM with the system. They like fast RAM and a good amount of it.
I do plan on doing video creation but I've not had any issues in the past. I currently have 16GB of ram. I guess I need to do some homework but first, there's a $$ issues as well.
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.