I just love temp questions...
Problem 1 is that they will never officially come out and say that product x can run max 90C
Problem 2 is that they should never have made temps visible to the consumer
Problem 3 is that in order to sell their product, they know that they can't sell a airplane prop as a fan, so they relax on the default fan speed settings resulting in a card that only responds to 70c+ temps
Solution to problem 1 and your request as to what is considered too hot: no one knows. But humanly. we all know that 95-100C is what is considered to be boiling point for water. That's inherent in our upbringing, and rightfully we are concerned with this.
So from what I've seen on the web, 80-90c is what is average for a x1900xtx using stock fan speed settings. Many people get this down to 70-80c using custom fan speeds. Using anything from 60-70% fan speed will keep the card below 80C
Solution to problem 2 and 3 is quite hard - but I have to state the following: If they can warantee a product that runs 90c in default, who are we to worry? As long as it's still under warantee, why care? Which brings me back to visible temp sensors - do you think anyone would be bothered if this wasn't a known variable?