Very few graphics cards on the market support a 32-bit z buffer. The DirectX SDK contains a large grid showing the capabilities of various cards. None of the offerings from ATI or nVidia listed support the D3DFMT_D32 depth/stencil format needed for a 32-bit z buffer. The only DX8/DX9 capable GPUs in the grid which do are the SIS Xabre and the XGI Volari, both of which are relatively low-end, niche products.