Lincecum and Hochevar will both likely start 2007 in AA, so they could be up in the bigs by midseason 2007, but that isn't too likely. It's even less likely that either of them would be "pitching effectively" - this wins my ambiguous term of the day award - by then.
Niemann has put in 77.33 AA innings this season, giving him a good shot at starting 2007 in AAA. He could be making starts at the big league level by May and become a pitcher to use right away.
I don't think you can really compare Petit's ceiling to the other guys who you listed, but as I'm sure you know, he's the most major league ready. I've never been on the Petit bandwagon - maybe just because I don't know enough about him.
So, I'd rank Niemann first under your criteria. This isn't the kind of question that I feel comfortable just ranking guys 1-4.
Mike Pelfrey > Matt Garza