Just installed a SAPPHIRE HD 4550 512MB DDR3 PCI-E into a Gateway SX series. I'm using HDMI cables. The problem is that I now get no signal from either the original (built in) video, or the new card. What could be wrong?
Check power supply, drivers, ram.. do a troubleshoot.
Also remember, when installing a graphics card you have to clean off the old display/video drivers from your previous graphics card. Reboot. Then do a clean install of the new drivers of your new graphics card while the new graphics card is plugged in.