Diego Costa would be a perfect upgrade on Giroud. I've not seen alot of Atletco this season (I only really watch R.Madrid games here and there), but he's pretty fast and strong. In our current formation, we need a player who can hold the ball up as there is only one striker, and to provide a different approach as we also need pace. Most of the goals Sturridge scores, are from a through ball which he has the pace to get onto. We don't have that really, nor does Giroud make any runs.
This is from their most recent game, individual highlights:
http://www.youtube.com/watch?v=GWeojIx55zM
Look at what he offers. He runs, runs and runs.