All GPU fans adjusted with the exception of the dual GTX295. Whenever fan settings changed (on VGA bios with NiBiTor 6.0.3), the card falls-off the bus (according to dmesg). Leave it alone for the time being.
GTX295's fan speed was finally adjusted by directly editing (with hexedit) the bios. The correct byte was identified through comparison with the GTX260 bios. Don't want to have to do this again …