16GB in a Dell Optiplex 3020 is worth it if you need it.

Yes, if you’re using it for basic office work, browsing, or light multitasking. DDR3 is cheap, and doubling the RAM will keep that old box from swapping like crazy. But it won’t fix slow CPU or lack of an SSD.

The Optiplex 3020 is a Haswell-era machine. With 8GB, it’s fine for one or two apps. With 16GB, you can actually have a dozen browser tabs, Excel, and Spotify open without stuttering. The CPU (i5-4590 or similar) is still plenty for daily tasks. The real bottleneck is the old hard drive—if you’re still on a spinning disk, spend your $15 on an SSD first.

So: If you already have an SSD, 16GB of DDR3 (used or new) is a cheap upgrade that makes the machine usable for another year or two. If you don’t have an SSD, do that first. If you have both? This machine is basically a free computer.