I did some digging and found that the F52 vBIOS update for my Gigabyte video card (GV-N640OC-2GI) supports UEFI.
In January 2014 I performed a clean install of Windows 8.1 on my year-old HP P7-1439 desktop computer. This machine originally came with Windows 8, but I installed Windows 7 Ultimate. So, I was making the round trip back to Windows 8 (actually 8.1). At the time, I was unaware of UEFI but eventually figured out the workaround. Install Windows 8 without the add-in video card, change the UEFI BIOS to enable legacy mode and disable secure boot. Then physically install the add-in video card and install the Nvidia software/driver.
This worked fine. But I wondered if Gigabyte offered a vBIOS that supports UEFI.
I looked here: http://www.gigabyte.us/products/product-page.aspx?pid=4255&dl=1#bios and used the VGA_@BIOS tool to determine that I had the F50 vBIOS. The site says to stay within the series when upgrading. So the upgrade is the F52. I emailed Gigabyte and they confirmed that this vBIOS supports UEFI.
After flashing F52, I can confirm that this vBIOS does indeed support UEFI. I went back to the UEFI/BIOS settings and disabled legacy mode and re-enabled secure boot.