In search of an impact arm, the Cardinals are exploring the possibility of trading for Houston Astros ace Roy Oswalt, and their chances are helped by the fact that Oswalt, a longtime rival, also has eyes for them.

The Cardinals are Oswalt's "first choice," according to a league executive with knowledge of what Houston told an interested team other than the Cardinals. At least one team that has spoken to Houston about the righthander has been told that Oswalt, who can veto any trade, expressed a preference for the Cardinals over the other contending teams he may consider.

Cardinals officials declined to discuss Oswalt, citing tampering rules that forbid them to talk about a player on another team.

"You want to have the (payroll) flexibility to add an impact (player) if it's possible," chairman Bill DeWitt Jr. said Thursday when talking in general terms about the July 31 nonwaiver trade deadline. "A deal for any premium player is a long shot. We are very reluctant to give up high-end talent (prospects). The marketplace has changed. I think there are few deals where you can be sure you're getting value back."

DeWitt and other Cardinals officials have said the team is in a position to add salary not just this year but in future years if it enhances the quality of player they could get at this year's trade deadline.

If a team is able to land Oswalt, that team would owe him the remainder of his $15 million salary this season and $16 million in 2011. He also has a $16 million option for 2012, and a report on ESPN.com said Oswalt wants his new team to exercise that option before he'll waive his no-trade rights. The Post-Dispatch reported Thursday that, according to a source, the Cardinals have been told Oswalt would discuss alternatives to the option if it would ease a trade.

Houston is asking a high price in return for Oswalt.

According to a National League executive, the Astros have said the package must include two top prospects, a third minor-league player, and a young player who is "major-league ready." The Cardinals have depth in the outfield, enhanced by the recent play of rookies Jon Jay and Allen Craig at the major-league level. The top prospect in the Cardinals' system, Low-A pitcher Shelby Miller, is a Texas native and a prototype Texas fireballer, but the Cardinals cannot trade him until he has been under contract for a calendar year. He won't clear that threshold until August, and if he's included in any deal he would have to be a "player to be named later."

Cardinals general manager John Mozeliak declined to disclose any specifics of the team's machinations as the deadline approaches, except to say there is a 'spectrum" of opportunities the team is considering.

Oswalt, 32, is 6-11 with a 3.12 ERA this season, and 15 of his 19 starts this season have qualified as quality starts. The righthander is 143-81 in his career with a 3.22 ERA, and only once has he made at least 30 starts in a season and had an ERA higher than 3.55. Facing the Cardinals in the National League Championship Series for the second consecutive October in 2005, Oswalt went 2-0 and pitched the Astros to the World Series with a Game 6 victory in the final game played at Busch Stadium II.

That World Series, which the Astros lost to the Chicago White Sox, was Oswalt's last appearance in the postseason.

Earlier this season, Oswalt's representatives — who could not be reached for comment Thursday — approached the Astros about the pitcher's willingness to be traded to a contending team. The Philadelphia Phillies have been linked to Oswalt, reportedly upping the intensity of their pursuit this past week when Jamie Moyer left Tuesday's game with an elbow strain. An online report from The Houston Chronicle said the Minnesota Twins have also shown interest.

In that same report, Astros owner Drayton McLane told the paper that the team has not taken "any type of proposal" to Oswalt.

"He's one of the best in the game. No doubt about it," Cardinals starter Adam Wainwright said when asked about the possibility of adding Oswalt. "That's no knock against anybody who is here now. He's just that good. He's going to help any rotation, and if he's in our rotation nobody — nobody — wants to play us in the playoffs. Not just regular season, but nobody wants to get in a postseason series with us."