SM3 has nothing to do with HDR support.
All of the Geforce 6 and 7 cards support full SM3. The Radeon X8 series and lower support SM2.0b which is basically an ATI made middle ground.
HDR on the other hand is completely different. You dont need SM3 for HDR. Also it depends on which implementation you are talking about.
- OpenEXR uses FP (Floating Point precision). Right now Nvidia uses 16bit FP (IIRC) and ATI actually uses something else (Which escapes me right now) which allows them to use AA along with HDR.
- Half Life 2 uses an implementation which is lower precision but all the calculations are done in the Pixel Shaders. The advantage is that you dont need support for Floating Point however you sacrifice some IQ
As for DX10. Microsoft claims that they recompiled all the code for DX9 and optimized it significantly. So while DX10 will run fine, DX9 will actually run better inherently on cards/software which use DX10.
-Kevin
(Oh and akshayt you have no idea what you are talking about. First off what is FPAA? the 16 you are thinking of is Nvidia's new AA detail level (16x). FP has nothing to do with AA)