It's a huge upgrade. If you're looking at benchmarks, it performs almost twice as fast on single thread tasks, and roughly three times as fast in tasks that can utilize multiple threads/cores. Keep in mind though - it's not a direct swap. The 3600X uses an AM4 socket and DDR4 memory, while the 9900X uses an AM5 socket and DDR5 memory. So along with the CPU, you'd need a new motherboard and new RAM. Depending on your power supply, you might need an upgrade there too, although the 9900X doesn't consume too much more power than the 3600X (120W vs 95W) so what you have will probably continue

The AMD Ryzen 9 9900X has integrated AMD Radeon Graphics, which can handle basic graphics tasks. However, for high-performance gaming, a separate gaming-specific GPU is generally recommended to achieve better performance.

Yes, the AMD Ryzen 9 9900X can handle gaming and creative work. It also handles standard productivity tools well. I coupled mine with the GIGABYTE X870 AORUS ELITE WIFI7. Couldn't be happier