I can't offer any help but I'm trying to upgrade the graphics card in my wife's HP slimline s3521p and have had the same issue. The default on-board graphics work fine but once I insert the new NVIDIA 9500 GT card and try to run the monitor is just stays black and eventually goes to power saver mode.
I have one of those that I pulled out of the boneyard and had the same issue. As an experiment I took the base plate off the card and put it in, allowing it to seat deeply in the pci express slot. Video works great now. Also make sure you go into the bios and tell the bios that the primary video is the pci express. Hope this helps guys.