What RAM speed should I use for a 7800X3D?

Aim for 6000MHz CL30. The 3D V-cache makes the chip less sensitive to RAM speed, and 6000MHz is the highest that reliably keeps the memory controller in 1:1 mode. Going higher often forces a 2:1 ratio, hurting latency and causing instability for tiny gains. Save the extra cash for a better GPU.