For high bit depth textures, use 16-bit or 32-bit textures rather than 24-bit textures.
24-bit textures do not fit neatly into cache. Using 24-bit textures can cause data to use more than one cache line and this has a negative impact on performance and memory bandwidth.
16-bit and 32-bit textures fit into caches without problems so they do not suffer from these performance issues.
For most textures, it is better to use texture compression than high bit depth textures. Compressed textures can improve performance because they use less memory bandwidth. For more information, see Use texture compression.