You can find a good review of the Houston system here, when they broke down the top 15 for the Astros.
Friday, January 20, 2012
You can never have enough prospect rankings
Head on over to Bullpen Banter, where they've posted their 2012 Farm System Rankings. Jeff Reese and Al Skorupa have each ranked the organizations, and both have the Astros right around the 33rd percentile.