I can guarantee to you that the X800 series does not support all of the features of Shader Model 3.

Because I've tried them. I've checked the caps, played around, nada.

ATI is a gigantic whore when it comes to their own proprietary shader extensions. They do extend things in non-standard ways, but mainly that's just a ploy to throw certain costly features into hardware, giving them a slightly higher performance boost in benchmark programs they can buy their standards into. Check the 3DMark2003 fiasco for instance. PS1 tests used Radeon 8500-only features and defaulted to much slower options for all other cards. They are not features you can use practically because there is no guarantee it will be available on all cards. Thanks to this sort of thing almost every production engine out there has a different pipeline for every damn card.