At what working temperatures will my gpu not only survive, but have a long healthy life

Solution 1:

<105°C

Junction temperature TJ usually is limited to +150°C for silicon ICs. Devices may operate momentarily at slightly higher temperatures, but device life expectancy decreases exponentially for extended hightemperature operation. Usually, the lower the junction operating temperature, the greater the anticipated life of the IC.

from http://www.allegromicro.com/en/Products/Design/an/an295014.pdf

There's a nice chart at http://www.interfacebus.com/Integrated-Circuit_Derating_Guide.html which shows 10 years at 108°C declining to 1 year at 148°C

The manufacturers quote a maximum temperature, for example 105°C for the GeForce 8800