That system should have an onboard video adapter. Remove the 6600 video card, and see if the system boots. If not, then something else is causing the problem.
If the system boots with the onboard video adapter, the go into BIOS, and look for an option to disable the onboard video adapter.
Also, make sure the video card is fully seated.
If those do not help, then either the card may be defective, or the PS may not have sufficient punch to run the system with that card installed.