Here at the Canadian Light Source (www.cls.ca) we use many Scientific Lin 
ux workstations with dual head configurations.  They are exclusively Dell 
Machines and I have been putting in NVidia Quadro FX and GeForce FX 5500 
video cards.  After downloading and installing the driver (very easy to do) 
I modify the 'device' section of the xorg.conf to implement the twinview 
function of the card, I can easily make a dual head machine.  See below for 
an example of before and after modifying the /etc/X11/xorg.conf.

Before;
Section  "Device"
	Identifier  "Videocard0"
	Driver      "nvidia"
	VendorName  "Videocard vendor"
	BoardName   "NVIDIA GeForce FX 5500"
EndSection

After;
Section "Device"

	Identifier  "Videocard0"
	Driver      "nvidia"
	VendorName  "Videocard vendor"
	BoardName   "NVIDIA GeForce FX 5500"
	Option	    "TwinView"
	Option	    "MetaModes" "1280x1024, 1280x1024"
	Option	    "ConnectedMonitor" "crt,crt"
	Option	    "SecondMonitorHorizSync" "30.0 - 80.0"
	Option	    "SecondMonitorVertRefresh" "56.0 - 76.0"
	Option	    "TwinViewOrientation" "LeftOf"
	BusID       "PCI:4:2:0"
EndSection

Hope this helps,

Carl Finlay
Research Technologist
Canadian Light Source