Another 8GB stick is the smarter move.

Yes, you can use a 16GB stick for 24GB total, but I wouldn’t.

The OptiPlex 3070 has two RAM slots. Adding another 8GB stick (same speed) gives you 16GB running in dual-channel mode. That means your CPU can talk to both sticks at once, which actually makes everyday tasks feel snappier—especially if you’re multitasking or using integrated graphics.

Slapping a 16GB stick in there with your existing 8GB gives you 24GB, but only the first 16GB runs in dual-channel. The last 8GB runs single-channel, and in practice the performance gain over 16GB dual-channel is minimal for most people. Plus, a single 16GB stick is more expensive than an 8GB stick, and you lose the ability to run two matched sticks later unless you buy another 16GB.

If you ever think you’ll need 32GB down the road, buy two 16GB sticks