click, them so hard...
With my two cards in a 64bit kernel, I could not get the nvidia module to load. /var/log/Xorg.0.log contained the (hopelessly ambiguous) "Could not open /dev/nvidia0 (Input/output error) that seems to be so common online. dmesg showed "NVRM: rm_init_adapter(0) failed" during the module load. With a little searching I found [this nvnews forum thread from over a year ago ] and turned off IOMMU which made the driver load and I was able to use both devices with four monitors. This was on an AMD (Asus) board.