Hello, I have an HP H8-1409 that I have upgraded extensively - upgraded 10GB 1333 memory to 16GB 1600 memory, OEM Hitachi boat anchor HDD with Kingston 240GB SSD and WD Caviar Black 1TB Storage drive, the OEM 460W power supply to a Corsair 650W modular, and the video card from the OEM HD7570 to an HD7870.

I would like to share my resolution of the video card upgrade.
I initially tried to upgrade to a GTX 660 but the screen just went black. The card tested out ok at the shop but I still exchanged it for the HD7870. That one booted up but the POST gave 6 beeps on boot up which indicated hardware failure. Returned the card which tested ok at the shop for a new card that also pre-tested ok at the shop. Same 6 beeps on start-up, after which it runs in a restricted mode. Went to HP support, they shipped the unit back under warranty, replaced the MB and updated the BIOS. Got it back, put the card in, same 6 beeps. Called their "special phone number" that was busy and left my number for callback. While I was waiting on the callback, I did some additional research.

Turns out that most/all W8 computers are shipped with "Secure Boot" enabled and "Legacy Support" disabled for security reasons. The answer is to hold down the esc key and repeatedly press the F11 key on start-up, then navigate to the setup page where you can reverse the settings: Enable Legacy Support and Disable Secure Boot.

Once I did this, I shut down the PC, swapped out the video cards, rebooted, and voila! Works like a charm.