Depends on the games you wish to play, truly...
ATI Radeon's have the performance crown right now for newer, shader-rich games as their X1800 and X1900 series have a walloping 48 pixel shader pipelines, but they are tad underpowered on fillrate for older games only having 16 pixel texturing pipelines (i.e. older games rich with heavy textures vs. shaders). ATI's can also perform HDR in combination with Antialiasing.
NVIDIA high-end cards are a little shy on shader muscle as they only have 24 pixel shader pipelines, but have incredible fillrate as they also have a full 24 pixel texturing pipelines for meaty texture games (i.e. ala doom3 engine games in "Ultra" texture modes, etc.etc.). NVIDIA cards cannot perform Antialiasing with HDR at this time as they do not provide a method of applying AA on render targets (just framebuffer AA).
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