reasonable GPU alert temperature

I recently bit the bullet and built a new machine: double Xeon configuration
with a GeForce FX5900 by Gainward. The CPUs run in the lower 40s celcius,
which is excellent, but the GPU on the graphics card runs a bit hotter.

After the system has been up for a while X-windows or win-XP, the GPU
core temp registers in the upper 50s. the driver software shows a default
core slowdown temp of 140 celcius and this seems WAY HIGH!

What would be a reasonable operating temperature for the GPU and at what
temp should I flag a temperature alert?

The Xeon CPUS automatically throttle back at 71 celcius. This seems like
a much more reasonable value.

Did some doofus who wrote the driver software put a farenheit value in
as a default that should be listed in celcius?

Any thoughts?