I don't know anything about the voltage stuff, but for temperatures, I generally use this page as a rough guide.
I've measured the temps of my GPU and memory on my Ti 4200 and have gotten a max of about 70 °C (about 158 °F) and about 36 °C (about 96.8 °F), respectively. I also have some of the Tweakmonster heat sinks for my memory. My GPU is at 300 MHz and memory is at 540 MHz.
I've been running fine with this for awhile now. Hopefully that page will give you a good idea of what types of temperatures these GPU's can put out. These things just get hot. That's basically all there is to it.