Trading Greinke and Zito for a hitter should allow you to get almost any elite player in the league. That's a lot of trade value, unless that team already has good pitchers.
I also own Greinke, and if I was going to trade him I'd expect a player like a Vernon Wells, Jason Bay or Ichiro. A good but not elite player. Greinke is 0-4, but look at his ERA and WHIP. He will get his going forward.
If you trade BOTH pitchers, I wouldn't settle for anything less than your choice of player, barring somebody like Pujols or Cabrera, etc. But you should be able to get an A-Rod, Braun, H Ramirez, or anyone you like. Quentin is so inconsistent right now you won't get much for him. I dropped him last week.
You might try Kelly Johnson and Greinke or Zito for Cano, Pedroia or Utley. That way they get a good 2B in return and you get an elite one. Utley would be my choice of the 3, although I'd rather give up Zito/Johnson than Greinke/Johnson.
It all depends on your league and how willing the managers are to deal and trade