Here are the links to the Houston Astro farm system. You can find all kinds of information and rosters there.
AAA: Round Rock Express
AA: Corpus Christi Hooks
A Adv.: Salem Avalanche
A: Lexington Legends
Short Season: Tri-City ValleyCats
Rookie Ball: Greeneville Astros
Rookie Ball: VSL Astros
From Baseball America:
Top 10 Prospects: Houston Astros
Houston was Baseball America's Organization of the Year in 2001, when we ranked their minor league talent the third-best in the game. The Astros system hasn't rated higher than 20th since—it currently checks in at No. 22—so owner Drayton McLane has opened his wallet to sustain the winning.
Because Houston hasn't drafted as well and has faced stiffer competition in Venezuela in recent years, the system hasn't been able to feed the big league club as it once did. While the top four home run hitters (Berkman, Morgan Ensberg, Craig Biggio, Jason Lane), leading winner (Oswalt) and closer (Brad Lidge) in 2006 were all homegrown products, precious little new blood has arrived on the scene.