This guy did an amazing job comparing the 2 processors. This helped me decide the 2.4Ghz is very very closely matched to the 2.66Ghz. Check out the benchmarks for the GPU 256 vs 512mb of vram its very close. I think that was already pointed out in another thread.
http://www.youtube.com/watch?v=z0-l37pgqa0&feature=related
http://www.youtube.com/watch?v=z0-l37pgqa0&feature=related