Particle board is also poor for load bearing spans. MDF (medium density fiberboard) is stiffer, denser, and acoustically as good or better than particle board (found in higher end speakers), and then 3/4" or thicker plywood is again stiffer and better for load bearing spans.
Originally Posted by avandesande
All should have some sort of stiffening battens if they span a significant gap in the support from below. MDF can be found at Home Depot in the US. Another option is strawboard (also found at Home Depot), made with more quickly renewable straw rather than trees, and without formaldehyde, a bit stiffer than particle board, but less so than MDF. MDF and strawboard also take paint and laminates nicely.