This is a bad recommendation. You’ll be dumping a lot of money into antiquated tech that can’t be upgraded further. Only upgrade things that you can take to a new high spec MOBO (hard drive, GFX card, PSU). It does make sense if you can get the components for dirt cheap, but a lot of times previous generation components are more expensive, not less, because they’re not manufactured anymore, and supply is limited.
It doesn’t have to match, but it’ll all run at the rate of the slowest stick. I just added a note about your PSU too, so check the bottom of my comment if you didn’t see it already.