AGP or PCIe, it still uses the same chip, and the same core voltage, with the same clocks and the same heatsink! At 100% fan duty mine ran 60c. I was surprised to see they(Sapphire, who makes my card as well) dont remove the plastic over the ram-pads. I was able to get HIS to send me their 1950 Pro's cooler (not telling you how, sorry lol Devise your own plan heh) and now I run 48-50c max load. But yes the stock heatsink is junk, get something else.
There IS a voltage table, just look in Ketxxx's 1950 thread. While I'm not using any of His BIOS' atm since I'm CPU limited I think, my core's max stable is 648 and memory is 864 (yes, 864). Just using the HIS Heatsink and memory-sink. All copper, very nice.
HIS (Had a x1600XT form them)