Wallace, the Cardinals 2008 first round draft pick currently plays for their Triple-A affiliate Memphis. The former Sun Devil is widely considered one of the top prospects in the Cardinal organization. John Sickels of Minor League Ball ranks him as the second best prospect in the Cardinals system. Furthermore, Sickels said Wallace has a "Monster bat and should be ready within a year."
Wallace has been performing very well at the Class-AAA level. He's currently batting .298 with six homers and 19 RBI's in only 32 games.
The Cardinals have been searching for a bat to protect Albert Pujols, and many believe Holliday would be the perfect compliment. Holliday's numbers are down somewhat this year (.287/.379/.453) from his career averages (.316/.385/.541), but considering he's in a pitchers friendly park they're still more than respectable.
The Cardinals are in the midst of a very tough pennant race in the NL Central, and acquiring a player of Holliday's caliber could be the move that puts them over the top. They certainly haven't been shy about making moves so far as they've already traded for infielder's Mark DeRosa and Julio Lugo.
The Cardinals probably will have to contend with both the Tigers and the Yankees to obtain Holliday's services, but I have to imagine if they're willing to give up Wallace they have a pretty strong shot.
Holliday signed a one-year $13.5 million contract this offseason, and he's scheduled to be a free agent after the season. Any team looking to add Holliday this season would probably be on the hook for about $7 million that is still remaining on his contract.