may be it sli only not 3way on that mobo
is there an older/newer driver you could use or may be try?
newer
http://www.nvidia.co.uk/object/win7_...2_whql_uk.html
i cant rember when the 3way drivers came out but there should be a 3way driver for the 8800gtx's do you have one on a cd that came with the gpu?