If you're going to have to drop anyone then I wouldn't pick up any of those guys. You're not going to keep them and they won't offer more production over the guys you have. Maybe over Adam Jones.
If you're going to take a chance then my order would be
Jennings Gordon Rizzo Ackley
Jennings offers a great power/speed combo. Gordon is 2 because of position, getting that speed and runs from SS is nice. I'm not on Rizzo's jock like everyone else. I think he will be good but no one can expect him to keep up those numbers in that park with that lineup...yuck. Ackley sounds like he will help in avg but his power hasn't developed yet.
This is for this year though, it would be a different order if it was a full keeper we were talking about. However, since you won't be keeping any of these guys at the end of the year then you're just looking for help now.
I lean Ackley. I'm not sold on Rizzo in Petco, plus he has not tammed the K rate, and may really struggle with the breaking ball in the major. Ackley is far from polished himself but is showing a much improved stat line and perpherials in the minors.
As was mentioned though, I probably wouldn't drop any of your guys for them in a 12 team league where you wouldn't be keeping them. If you get some sort of extra rookie keeper then by all means go for one.
I think I'd go with Rizzo, if he gets off to a hot start, use him as trade bait for maybe another solid pitcher or package him with Berkman and get a better OF/SP combo. People are really high on Rizzo right now, and you could take advantage of that. You wouldn't keep the guy anyway, so using him may give you a better keeper.