I bought my brother a video card (1050 Ti SC) and when installing it in the computer, it does not go past the Bios Step.
 
- The CPU fans turn on, it starts beeping and displays a 'Cannot access the PCI slot' (or similar message, I'm paraphrasing.)
- The PSU is < 2 years old, and has a 650W rating.
- This was attempted with the onboard video on and then with it off, with the old video card alongside and without the old video card.
- The PC boots normally with the old video cards. (2x 9500GT SLI)
- We used the 3.0 16x slot that is 2nd from the CPU (The large yellow one)
 
Any help would be appreciated :)

Updated the BIOS on that mobo?

Luta Wicasa
Updated the BIOS on that mobo?

This is likely the cause of your issue. 
 
Please also try clearing CMOS as well.

So, I walked him through updating the bios. That did something, it now boots, you can hear windows boot, but now trying three different setups, the video won't display to the monitor:
 
1. Onboard video off: New Card in.
2. Onboard video on: New Card in. (Neither HDMI in Mobo or card outputed)
3. Onboard video off: Old cards in.
 
 

You cleared the CMOS?

Resetting the CMOS, allowed for the old setup (2x 9800GT SLI) to work.
Video will not display (even to get into the BIOS) for any other configuration. (New 1050 card, onboard video, old cards sans SLI).
 
(Edited to clarify).

OK, so problem was solved.
 
I think it was the simple issue of using HDMI before installing drivers. But these are the full steps (including those above) taken:
 
1. Updated the BIOS
2. Resetted CMOS
-> Due to not working, the original cards were put back in.
3. Downloaded and Installed the updates for the Intel Chipset, and the Onboard Video.
4. Went into BIOS, Turned on Multi-Monitor Support for onboard video.
5. Hooked one monitor to the VGA port of onboard video.
-> Computer booted with onboard video functioning (original cards still in PC)
6. Took out original cards, put in EVGA 1050 in the 1st 16x PCI 3.0 slot.
-> Computer booted, onboard video not functioning, this time I hooked up a monitor using a DVI cable. Video worked.
7. Installed Nvidia drivers.
8. Rebooted, hooked up second monitor via HDMI.
-> Both monitors worked.
 
Also of note: Originally, I was posting here, waiting for replies, and troubleshooting over the phone. I drove down to my brother's place today. I didn't know until then that he was using only HDMI.
But thanks for the help.