For RAM I like suggesting A-Tech.  Less costly than Crucial, but has a lifetime warranty. I'm with redxps630 on upgrading to an SSD.  My favorite SSD adapter:  Orico SSD adapter on Amazon  Beware of cheapy adapters that can leave hanging tab in the way of connections.  Been there.
Wanting better performance.
"DS: On an older Optiplex that you want to upgrade, what is your go to part(s) 
replacement