So far in his career, Zito's pre-all-star break era is 4.63, with a 2.49 mark afterwards. Lots of people gave up on him after a rough start last year, and kicked themselves when he put up unbelievable numbers down the stretch. I think he'll be fine, although I might consider sitting him early on when he faces tough opponents.
Still, much as I like Zito, I think giving up two of the guys you listed is too much, especially at this point, unless there's somebody on the free agent list who you want anyway.
I'd counter with a slightly lower offer (Ritchie + Park?), or wait and see if Zito's owner becomes frustrated enough to deal him for peanuts (last year, he was waived outright by a large number of owners). With luck, you'll be able to get him at a better price than this.