I do not think you can do that. You cannot have two graphic cards running at the same time (unless its a ATI or Nvida Cross fire where you can do that). You need a graphics card that has two slots to install monitors like the ATI X1800XT (http://www.110220volts.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=100435705) . You can see from the image that it has those two white slots (DVI connectors) in the back that are for monitors. Unless you graphics card has that then there is no way to do what you are doing. Also, your motherboard VGA slot is disabled when you installed a graphic card. The reason why it is disabled is because there would driver comflicts with your graphic card and the graphics card on your motherboard.

I think your ATI has one of VGA and one DVI connector. If that is the case, I think your ATI should have came with a DVI to VGA coverter that will let you connect your VGA monitor into the DVI slot. Then try to extend as you were doing before.