Nice deal for both teams, I think Garza is going to be a stud. And Young has All Star potential no doubt, and greatly helps that Twins O.
If I were the Twins, I would keep Santana if that trade goes through. Losing Silva and Garza will hurt, but the Twins have an excellent bullpen, and their starters would not be horrible - Santana, Bonser, Slowey, and Baker. And if Liriano can be half the pitcher he was, they are in great shape.
As for hitters? They would still need some help, because outside of Morneau, Mauer, Cuddyer, and Young, they would be pretty bad.