Well... no opinions here, but the Q8400 is going to be better "most" of the time.
Programs that are only dual core optimized will see the E7500 perform better because of the 1MB extra L2 cache per core and higher clocks. (3MB L2 vs 2MBx2 L2, 2.93ghz vs 2.66ghz) The E7500 will also run cooler and use less power (which in turn leads to better overclocking), since it is a dual core.
So for "most" people/situations the Q8400 will be better.