Sorry if this also fits into the category of "things checked long ago," but could you post your xorg.conf?
Section "ServerLayout"
Identifier "aticonfig Layout"
Screen 0 "aticonfig-Screen[0]-0" 0 0
Screen "aticonfig-Screen[1]-0" RightOf "aticonfig-Screen[0]-0"
Screen "aticonfig-Screen[2]-0" RightOf "aticonfig-Screen[1]-0"
Screen "aticonfig-Screen[3]-0" RightOf "aticonfig-Screen[2]-0"
EndSection
Section "Module"
Load "glx"
EndSection
Section "Monitor"
Identifier "aticonfig-Monitor[0]-0"
Option "VendorName" "ATI Proprietary Driver"
Option "ModelName" "Generic Autodetecting Monitor"
Option "DPMS" "true"
EndSection
Section "Monitor"
Identifier "aticonfig-Monitor[1]-0"
Option "VendorName" "ATI Proprietary Driver"
Option "ModelName" "Generic Autodetecting Monitor"
Option "DPMS" "true"
EndSection
Section "Monitor"
Identifier "aticonfig-Monitor[2]-0"
Option "VendorName" "ATI Proprietary Driver"
Option "ModelName" "Generic Autodetecting Monitor"
Option "DPMS" "true"
EndSection
Section "Monitor"
Identifier "aticonfig-Monitor[3]-0"
Option "VendorName" "ATI Proprietary Driver"
Option "ModelName" "Generic Autodetecting Monitor"
Option "DPMS" "true"
EndSection
Section "Device"
Identifier "aticonfig-Device[0]-0"
Driver "fglrx"
BusID "PCI:3:0:0"
EndSection
Section "Device"
Identifier "aticonfig-Device[1]-0"
Driver "fglrx"
BusID "PCI:6:0:0"
EndSection
Section "Device"
Identifier "aticonfig-Device[2]-0"
Driver "fglrx"
BusID "PCI:5:0:0"
EndSection
Section "Device"
Identifier "aticonfig-Device[3]-0"
Driver "fglrx"
BusID "PCI:4:0:0"
EndSection
Section "Screen"
Identifier "Default Screen"
DefaultDepth 24
EndSection
Section "Screen"
Identifier "aticonfig-Screen[0]-0"
Device "aticonfig-Device[0]-0"
Monitor "aticonfig-Monitor[0]-0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
EndSubSection
EndSection
Section "Screen"
Identifier "aticonfig-Screen[1]-0"
Device "aticonfig-Device[1]-0"
Monitor "aticonfig-Monitor[1]-0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
EndSubSection
EndSection
Section "Screen"
Identifier "aticonfig-Screen[2]-0"
Device "aticonfig-Device[2]-0"
Monitor "aticonfig-Monitor[2]-0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
EndSubSection
EndSection
Section "Screen"
Identifier "aticonfig-Screen[3]-0"
Device "aticonfig-Device[3]-0"
Monitor "aticonfig-Monitor[3]-0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
EndSubSection
EndSection
I have 3 gentoo boxes running 2x7950, 2x5830, and 1x5770/1x7950 with minimal problems in the setup if you're looking for a different distro.
Got a step-by-step guide we can follow?