How hot is too hot? (Or, how my graphics card burned my fingers)

"Well, the problem ended up being the motherboard. After spending an hour on the phone with ASUS, they insisted that it was everything but the board. They finally settled on the power supply, insisting that it was insufficient, even after I pointed out that it ran fine for two years. I verified my hunch by putting in a Gigabyte GA-EP45-UD3P (well, actually two, the first one had defective LAN ports) and it works perfectly fine.

The power supply, an Antec Earthwatts 380 watt unit is plenty for my needs, according to http://extreme.outervision.com/psucalculatorlite.jsp and it never once hiccuped while the system was under stress from four instances of Prime95 and ATITool's artifact finder. They absolutely refused to acknowledge that there might be a short in the board due to the issue only happening when it is cold. After that kind of rude behavior, I will never recommend them again.

Thanks for everyone's help!"

The ATI 4800 series of GPUs are notoriously hot, and the reason is that the auto fan control defaults to keeping the system quiet rather than keeping the GPU cool, so they tend to idle around 75ish. If you install the latest drivers from ATI.com with the catalyst control center you can manage the fan speed manually. I have my 4870 set to a standard 40% fan speed with no issues at all and it never gets above 65C, even under extended load.

Could installing the new CPU have changed the speed of the (unspecified) bus that the video card is plugged into? Check your BIOS settings for bus clock speed.
Could installing the new CPU have changed the speed of the (unspecified) bus that the video card is plugged into? Check your BIOS settings for bus clock speed.