Def look to improve your pitching first. Mussina's droppable, your other starters are doing okay, but every one is a question mark. Look for some young starters and the next time a closer goes down, grab his replacement. (But don't drop Marmol -- even without saves, the guy is valuable.)
On offense, I don't understand why you're benching Chris Young. He'll hurt your average, but I think he'd contribute more overall than Theriot. If you're not going to play him, you could think about trading him to help your pitching. Similarly, I like Peralta enough that I'd think about trading Michael Young to boost your pitching (if you can get a good offer). Iwamura is an okay player, but he seems droppable.