Is the Ryzen 9 9900X3D good for gaming?

Yes. Its 3D V-Cache gives it a large gaming advantage over non-X3D Ryzen chips. It trails the 16-core 9950X3D slightly in some titles but costs far less.

Do I need a BIOS update for this CPU on the B850 board?

B850 boards ship with AM5 support out of the box, and the 9900X3D is a current-generation part. Check the board's support page for the minimum BIOS version before assembly.

What memory does this combo take?

DDR5 only, in the board's DIMM slots. AMD's sweet spot for Ryzen 9000 is around 6000 MT/s with low latency, and EXPO profiles make that a one-step setting.

Will a micro-ATX board fit my case?

Micro-ATX fits any case that lists micro-ATX or ATX support. Check clearance for your GPU length and CPU cooler height, since compact cases are tighter.

Does the board have Wi-Fi?

The listed connections are USB-C, USB-A and an onboard network card, with no Wi-Fi mentioned. Plan on Ethernet or a separate Wi-Fi card if you need wireless.

Is this combo worth it over buying the parts separately?

A bundle saves you matching a board to the CPU and usually simplifies returns. Compare against a TUF or ROG B850 board if you need more M.2 slots or better power delivery.