My Ti4400 GPU stays pretty hot the whole time - it will only go up by ~3-5C when under full load - that's under full CPU load as well- so the water temp is raised a bit too. When on a blank screen(power saving) the temp will drop another degee or so but not much.
I'm going to have 2 gallon reservoir. I'll try to keep the reservoir temp at around 10C. (Just above the dewpoint in my computer room.) Ideally, the pelts would consume low power keeping the water cold most of the day while the CPU and GPU are idle. Then when I wanted to play for a few hours, the pelts would have to consume more power keeping the water near 10C.
It sounds like the GPU heat drops some when the PC is idle, but not as much as I was hoping.