What affordable upgrades would make the biggest difference in this PC?

If you have access to a good local hardware retailer, an AM4 bundle with a Ryzen 5 5600X3D, a newer B550 motherboard, and DDR4 memory could be worthwhile for roughly $300. You could sell whichever RAM kit you don’t need. For the graphics card, the best choice depends on your budget: a used RX 6700 XT is a good sub-$300 option, while a newer 16 GB card around the $450–$500 range would provide more longevity. A used RTX 3070 Ti is another possibility, though buying used requires checking the card’s condition.

Because your monitor is only 1080p at 60 Hz, you don’t necessarily need an expensive GPU upgrade right away. A Ryzen 5 5500 would still be a meaningful improvement over the 1600 if it’s very inexpensive, but a Ryzen 5 5600 is the better target. The GTX 1660 Ti should remain adequate for 60 FPS gaming in many titles, so I’d upgrade the CPU first and consider a graphics card later if you move to a high-refresh-rate monitor or start playing more demanding games.

A used Ryzen 5 5600 or 5600X would be a strong first upgrade for around $100 or so. Your B350 board may support it, but check MSI’s CPU support list and update to the required BIOS while the Ryzen 5 1600 is still installed. After that, a used RX 6700 XT with 12 GB of VRAM would be a noticeable graphics upgrade if you can find one around $250–$300. Those two changes would make the system feel substantially newer.