The X1600 Pro has SM3.0 and FP16 support for HDR... but has 4 pixel pipes, 16 pixel shaders, 5 vertex shaders. It also only has a 128-bit memory interface.
In a nutshell, while it has pixel shader performance similar to the X800 GTO, it has less fillrate and less bandwidth. It basically trades a large degree of performance for a couple of extra features. Still good cards though!
The X800 GTO is designed to be a SM2.0/I16 blend card. Stock, these have full 12-pipelines (12 pixel shaders as well 1:1) plus a 256 bit memory interface.
The Sapphires linked are a special, 16-pipe model... so as while they do not support SM3.0 or FP16 blends for some forms of HDR, they have blindly larger scale power and much, much greater performance.
You wont be able to enable HDR in Oblivion, but you can use Bloom as well as several "Fake HDR" mods to yield similar effect. In the end, you'll be able to play Oblivion with much greater framerate and higher resolutions.. not to mention crank EVERYTHING else up higher (except HDR) and use anisotropic filtering to sharpen the game up for basically "free"..