run the two 7900's in SLI mode and the 7950's in single card mode.reason for running 7950 in single card mode is that nvidia is still having some issues with the drivers for that card in sli.still getting its pants beat off by even single card 7900gt in some instances.
Dahak
EVGA NF4-SLI MB
X2 4400+S-939
2X1 GIG DDR400 IN DC MODE
2X7800GT IN SLI MODE
520WATT PSU
WD 300GIG HD
EXTRME 19IN.MONITOR