This is not quite perfect, but as a rule of thumb, The Core2 based designs run, clock for clock, about 25% faster than the x2 based AMD designs.
1.66 x 1.25 => 2.075 versus 2.0, s over all , pretty close.
The AMD chip is cheaper, so IF you can turn the savings into more Ram, better HD, better video, etc the AMD may be a better deal, but raw cpu speed is pretty close.