Sheldon Brown's site has excellent tables showing the specific cog sizes for cassettes from the big players -- e.g., the XT & XTR 11-28 8s cassettes have different cog sizes than the cheaper HG50 cassette. Not just that, but also information about tooth transitions, spider assemblies, etc., that can help you customize a rear cluster from standard cassettes.

Here's the 8-speed cassette page.