Ok, this is not meant to be real exhaustive, but looking at the people currently projected in some way on the 25-man roster for the Astros:
1B: Lance Berkman - Drafted by Houston
2B: Kaz Matsui - Signed FA
3B: Ty Wigginton - Traded Dan Wheeler
SS: Miguel Tejada - Traded Package B
LF: Carlos Lee - Signed FA
CF: Michael Bourn - Traded Package A
RF: Hunter Pence - Drafted by Houston
C: J.R. Towles - Drafted by Houston
Bench:
Brad Ausmus - Been with team since 2001 for 2nd tour. Acquire via trade both times.
Mark Loretta - Signed with team as FA
Darin Erstad - Signed with team as FA
Geoff Blum - Signed with team as FA
Starters:
Roy Oswalt - Drafted by Houston
Brandon Backe - Traded Geoff Blum (irony, but Blum not Astros Farm product)
Wandy Rodriguez (signed by Houston as NDFA, so from Farm System)
Woody Williams - Signed with team as FA
Chris Sampson - Drafted by Houston
Bullpen:
Jose Valverde - Traded Burke (Astros Draftee), Gutierrez (Astros System, NDFA), Chad Qualls (Astros Draftee)
Oscar Villiareal - Traded Josh Anderson (Astros Draftee)
Geoff Geary - Traded Package A
Doug Brocail - Signed with team as FA
Mark McLemore - Drafted by Houston
Wesley Wright - Acquire via Rule 5 Draft
Package A: Brad Lidge (drafted by Houston), Eric Bruntlett (drafted by Houston)
Package B: Troy Patton (Drafted by Houston), Luke Scott (acquired via trade for Jerome Robertson who was drafted by Houston), Matt Albers (Drafted by Houston), Dennis Sarfate (Waiver pick-up), Mike Costanzo (acquired via trade in Package A)
That leaves two roster spots open, a position player and a pitcher (bullpen/starter).
So of the 23 people I just listed, 6 were drafted by Houston and have only played in Houston. 7 were acquired via FA signing. And the remaining 10 were acquired via trade. Only 3 of those trades (adding in Ausmus) wasn't acquired by trading only (or almost only) players drafted by Houston, thus in my book they were still added because of the Farm System.
So that is 13 of the 23 that are direct results of players the Astros have drafted. And of the remaining 10, 7 were FA add ons which is pretty common for bench/bullpen types which is where most of them are.
To me that means the farm system is doing its job of providing Major League talent to the Major League level.
The reason I got on this little side issue is the constant hammering the club has been taking recently over the poor state of the farm system, yet last year it produced Hunter Pence and this year is producing J.R. Towles. AND the talent provided the method through which they acquired Miguel Tejada, Michael Bourn, Jose Valverde, Geoff Geary and Oscar Villiareal.
That is quite a bit of talent given how "poor" the sytem is supposed to be, right?