If you head on over to baseballprojections.com, you can find the CHONE projections for many, many players.  CHONE is nice because it's cheap and named after a guy Jim Hendry didn't overpay to get.

First off, note that the Cubs come out ahead on the Runs versus Replacement category but behind in the ERA category.  There are two reasons for that.  First, that's because CHONE is working a park effect into the ERA projection, but leaving it out of the RvRep.  It's easier to pitch at Busch than at Wrigley.  Second, RvRep is a counting stat, which means that Ryan Dempster counts for equal value as Chris Carpenter, despite Carpenter's projected ERA being much better, even accounting for park effects.  The reason is that CHONE doesn't expect Carpenter to pitch a full season.

So, what do we see here?  There's a lot of irony for me that CHONE expects pretty much the same thing for the much-hyped Found Penny and the afterthought of Tom Gorzelanny.  Then, in front of those guys, the Cubs front three (Z/Demp/Ted) exceed the Cards' three (Carp/Wwrt/Lohse), mainly on account of the inning expectations.  That leaves us comparing Randy Wells to Rich Hill, which is interesting, because CHONE seems awfully optimistic on Rich and fairly pessimistic on Randy.  Bill James and Marcel are both more optimistic for Wells.

But let's stick with CHONE, because even with the pessimism on Wells, even with the optimism on Hill, even expecting a Guzman injury, even dinging Marshall's ERA as if he were to continue as a swingman, taking all that into account, the Cubs are keeping pace with the Cards.

But you can see why the Cubs want to add a veteran RH reliever.  (On the other hand, you can also see why DCF keeps pushing his anti-Grabowism, since Gaub doesn't project far off from Grabow; so the Cubs will want to make sure they find someone better than what we have.)

UPDATE

Alright, I'm adding Bill James projections for the rotation since there's been some whining about CHONE's projection for Chris Carpenter.  I tend to take Bill James' projections less seriously than CHONE's, but the main reason to use him here is that he gives Carp 195 IP and Wainwright 225 IP.  Taking the Cubs and Cards rotations on a whole, James gives the Cubs a 1.33 WHIP and a 3.88 ERA, while giving the Cards a 1.31 WHIP and a 3.90 ERA (although I added and divided those myself, so someone may want to check my math). There are some issues here.  Penny's projection was made as a Giant, but I think SF's park effect is more pitcher-friendly than StL's.  Someone correct me if I'm wrong and a significant adjustment needs to be made.  James also doesn't project Rich Hill.