Is 32GB at 3200MHz stable on a B450 motherboard with a Ryzen 7 2700X?

Probably not out of the box. The Zen+ memory controller struggles with 32GB at 3200MHz, especially with dual-rank sticks. You might boot at 2933MHz or 3000MHz, but stable 3200MHz is a coin flip. If you haven’t bought the RAM yet, grab a kit from your board’s QVL list or settle for 3000MHz.