Keith Law listed his Top 100 prospects (Insider-Only) for 2012 and we find the following of particular importance:
#46 - Jonathan Singleton
#60 - George Springer
#78 - Jarred Cosart
Notably, Anthony Gose was #59.
And here (also Insider-Only), Law ranks the Astros' farm system as a whole at #27. Ranking below the Astros are the Marlins, Indians, and White Sox.