Is the Ryzen 9 7900 worth it over the 7900X?

For most buyers, yes. It delivers close to the same multi-core performance at a much lower 65W power draw, so it runs cooler and quieter. The 7900X is only worth it if you need the highest clocks.

Do I need a graphics card with this CPU?

No for basic display output, since it has integrated Radeon graphics. You do need a discrete GPU for gaming or GPU-accelerated work.

What motherboard and memory does it need?

It uses the AM5 socket with DDR5 memory. Pair it with a B650 or X670 board; a BIOS update may be needed on older boards.

Does it come with a cooler?

Yes, the boxed version includes AMD's Wraith Prism cooler. It handles stock operation, but a larger air or liquid cooler is better for sustained all-core loads or overclocking.

Yes. The multiplier is unlocked, and Precision Boost Overdrive lets you raise power limits for more performance if your cooling allows.

How does it compare with the Ryzen 7 7700X?

It has four more cores and eight more threads, so it is clearly faster in multi-threaded work. The 7700X is cheaper and close in lightly threaded tasks.