The diode is a thermal diode, basically its like a thermometer that measures the temperature. Its embedded in the CPU, but some motherboards don't take advantage of it, but rather rely on an on board sensor/diode to measure the CPU temp instead. I can't remember if the K7S5A is one of them though. The temps aren't meant to be extremely accurate, but just give you a general ballpark. They are repeatable and consistent within the same system, but just don't compare it with other people, it wouldn't be comparable/accurate.
As for you GF3 temps, the motherboard doesn't support anything like that, so the only way you could find out the temps would be to attach your own thermal diode or other temp sensor to the GPU and fix some sort of display LED/LCD so you can tell the temp - sorta clunky but it'll work.