Renteria would be a good fit because they need someone at the top of the order, he'd fill the hole at shortstop, and he was a good fit there last time.
The next need is the starting rotation. To compete with that division, the Cubs need someone beyond Lohse and Wainwright even if Carpenter is healthy. Wellemeyer will never be more than a midrotation starter because he gives up too many flyballs.
They don't need to sign an ace, but they need depth. They can't rely on five starters to get them through the year. They could sign a big name like Burnett, who would legitimize that rotation. If they feel confident with their starters, they could sign a lesser innings eater like Jon Garland, who will pitch long enough each game so that they wouldn't need the bullpen as much. Or they could sign a veteran with playoff experience like Randy Johnson.
The Cards need lineup protection behind Pujols. Ludwick, Glaus, and Ankiel don't make for a deep lineup beyond Pujols. While they already have four outfielders capable of playing every day, they could afford to sign another corner outfielder as there are many of them with big bats on the open market.
One possible bat is Milton Bradley, who they should deeply consider. He's a switch hitter and he lead the AL in OBP. He doesn't need to play every single game because of the Cardinal's outfield depth. A lesser name is Rocco Baldelli, who can't play every day, but doesn't need to.
The last need is the bullpen where they could use a few relief pitchers. For closer, I would recommend Trevor Hoffman. He won't cost much and the Cards won't give away draft picks to sign him unlike K-Rod, Fuentes, and Wood. I wouldn't be surprised if they signed a big name at closer, seeing how they paid so much for Isringhausen, but I wouldn't recommend it. There's a lot of middle relievers they could sign as well.
Ideal Roster:
C: Yadier Molina
1B: Who do you think?
2B: Aaron Miles
3B: Glaus
SS: Edgar Renteria
OF: Ludwick
OF: Ankiel
OF: Colby Rasmus
Rotation:
Wainwright
Lohse
Jon Garland
Carpenter
Wellemeyer
Piniero
Closer: Hoffman