Something is very wrong if you cannot run DXDIAG. DirectX should not have been corrupted by the installation of a video card, unless you did not remove the old drivers for the previous video card before you installed the ATI card.
Also note, if the previous video card is an onboard chip, you must remove the drivers for it, and upon the next reboot, immediately go into the system BIOS ROM and disable the onboard video chip. If you failed to do that, then you could have the problems you are describing.