What would you upgrade in this PC with $600?

For gaming, the graphics card is the first place I would look. The 5900X is still a capable CPU, and the rest of the build doesn't need replacing just to support a stronger card.

Check current prices on the RX 9070 and 9070 XT, plus the RTX 5070. The XT may be over budget unless you sell the 6650 XT or find a good deal. Compare benchmarks for the games and settings you actually use, rather than assuming every game will improve by the same amount.

Is there a game that isn't running the way you want? If everything feels good at 1080p, there's no need to spend the $600 now. Save it until you have a specific performance problem or a monitor upgrade in mind.

Think about the monitor alongside the GPU. A 1440p high refresh display would give you a sharper image, and a stronger card would help drive games at that resolution. The RX 9060 XT 16 GB is one option to price alongside a monitor, though you would need to check whether both fit your budget.

Moving to 1440p puts more work on the GPU. It doesn't make the CPU faster, and a 1080p 144 Hz screen can still benefit from a better card in demanding games.

If your main goal is higher frame rates in competitive games, check where the limit is before buying a GPU. Watch CPU and GPU usage while playing the games you care about, and compare the frame rate with your 144 Hz display.

A graphics upgrade helps most when the GPU is the constraint. If you're already getting the frame rates you want, a CPU swap on this build may not be a good use of the money either.