If they really are willing to do it just for the expirings (I am not 100% convinced of this...Diaw could actually be of help to them down the stretch), then I would do this deal in a second.
Farmar has been good this year, and he would be a big upgrade over Dooling and can play next to Bradley (he also may opt out of his deal after this year, because he played so well this season), and Petro is a servicable big man, who would be fine to have on the roster next year at that price.
But again, I think these deals are not always as cut and dry as they are portrayed. It looks like its just about the expiring contract...and yes, that is absolutely a key...but the Nets are still only 4.5 games out of 8th place, and could use a guy like Diaw the rest of the season.