From my experience, in most games you don't want to run them with v-synk disabled as there are quality defects, otherwise it only needs to be as high as the refresh rate and both cards can do that at everything but the very high resolutions where the Radeon wins.
I would get the Radeon mostly for imroved visual quality and more features like its DVD support.
But for framerates, does anyone know how to enable HSR (Hidden Surface Removal) as I saw a review once which showed sugnificant increases in framerates, as fast as a GF3 if not faster but with some visual artifacts (missing surfaces) at the higher tiling setings.