OK, I've done some more tests. Turns out the fan does spin dinamically if no profile is loaded in nvidia's System Tools (former nTune). However, if the fan is left on its own, it doesn't spin fast enough to prevent 80C, in fact it seems to consider it a fair temperature (as I would, considering it's GTX280). But unfortunately, that temp is enough to cause my problematic crysis artifacts.
BTW, FurMark does raise the fan to 100% and still the temp climbs to 89C. No artifacts or any problems whatsoever. Only crysis seems to be affected as soon as it hits 80C.
Is anyone from nvidia reading this thread? Is this situation normal? Please help.
EDIT: I've re-run crysis with dynamic fan and this time it went to 100% and kept temps at about 75C, preventing the artifacts but ruining my mind with the noise... Upon a fresh reboot though, the fan stays at about 60% and takes a long time to go beyond that (running furmark beforehand helps, like I did before