The 2.66 is definitely worth the money to anyone buying a Mac Pro. The 3.0 is only worth the money if the number 3 psychologically makes you feel better about your purchase.
running native apps the 2.0 would best the 2.5 g5, not by much but it'd be faster and it'll last longer, but if you can afford a mac pro 300 bucks more is not much for such a large performence leap.