Depends. Nvidia cards have some thermal protection, but whether that would protect from a TOTAL fan failure is anoher story. The Nvidia cards back-off the speed when they deect overheating. But I think they need a fan for regular gaming even if not taxed or underclocked. The Screensaver overheating issue should be an example of that, where even a screensaver which 'tricks' the card into turning off the fan can cause serious overheating. So on a high level card a fan is a must. I don't know of any built-in 'kill' switch/device to protect against vpu/gpu overheating, and ATI has no onboard thermal detection device. I think they all should considering their price and current clock speeds / heat production.
The other problem is that no one wants to test this in the review sites. They wouldn't even test the screensaver bug for fear of blowing the one card they had (a new [at the time] FX5800U) which would have been only a $400 loss for a pretty interesting test IMO.
I'm sure that there are people with 3rd party apps out there hooked up to temp sensors (or onboard) that may hav a system kill switch, but I don't know.
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