How Will Upgrading My GPU Affect Performance If I Switch to a 1440p Monitor?

You shouldn't expect a significant performance drop with that upgrade. The jump from the 6600 to the 7800 XT is quite substantial, so it should handle 1440p gaming pretty well, especially if you keep ray tracing off.

Keep in mind that 1440p has about 1.7 times more pixels than 1080p, which might lead to some performance decrease. But with your new GPU, it shouldn't be crippling. Just consider what games you're playing and their settings.

If you check out benchmarks, your 6600 typically averages below 40fps at 1080p, while the 7800 XT can hit around 70fps at 1440p. So, even with the resolution increase, you should still see a solid boost in performance.