The 2.66 quad MP is comparable to the Dell Precision T7500 with a quad Xeon 2.66.
The situation is more complicated than that. The quad Mac Pro uses a W3520 so is comparable to the Dell Precision T3500 which is about 40% cheaper, but
it has power supply etc that is larger (the same as the octo) which are
comparable to the T7500.
The T7500 only uses the more expensive 5500 series chips comparable to the octo MP. the 2.66GHz 5550 is much more expensive than the 2.66GHz 3520 (it is about 3 times the price) so it is important to know which Xeon when making
comparisons.