My legacy ECS K7S5A motherboard BIOS hardware monitor says the 5v is
low. So I used a multimeter to check a USB 5v header pin, and it says
4.75v.

The power supply 5v disk connector has exactly 5v, so the power supply
is good.

If I replace the 5v AGP card with a 3.3v card, the USB pin measures
4.85v. So the 5v video card causes a 0.1v drop on the motherboard 5v.

Is 4.75v PCI/USB safe? Should I replace the AGP video card to get the
extra 0.1v? I hear that 5% tolerance is OK, but 4.75v is right at the
edge of 5%.

The motherboard seems to work OK, but I wonder if it's safe.