XFree does not support DRI acceleration on multiple graphic cards. The only way to get multiple accelerated X sessions is to use Nvidia's closed source driver and GL library or a single card using DRI and multiple cards using Nvidia's closed source drivers. XFree extensions not dependent on DRI should work.
Most of the multiheaded graphic cards can be used only for a single user/display. In order to start independent X servers the heads have to be explicitly addressable (which can not be done with most of the cards). It should be possible to use cards with explicitly addressable heads for independent displays, but this has not yet been tested.
In the following cases it might be possible to use a single multiheaded card for multiple independent displays :
The card(s) have different PCI Bus ID's for the different heads
(for example Matrox MMS G200/G450)
The card(s) register frame buffer device for each head (only with the Linux-2.5/2.6 patch)
Have in mind that the XFree frame buffer driver does not support acceleration and most of the XFree extensions.
(for example Matrox G400DH, G450DH, G550DH)
Using/configuring independent devices for the independent screens/users is pretty hard or not explored. Exceptions are the input devices, graphic cards and sound thanks artsd. Some examples of such unexplored areas:
External storage device (USB/Firewire hard disks, CD/DVD drives, ZIP/Floppy drives, memory sticks, ....
USB/Firewire printers, scanners, cameras ...
In case you succeed in configuring such devices for independent usage by multiple users, please share with us how you did it.
This doesn't mean that the devices won't work, but that every user will have access to all devices.
Currently under Backstreet Ruby/ Ruby you can have a maximum of 16 Virtual Terminals (8 for Backstreet Ruby released before 7 Oct 2003). This means that without hotplug configured if you are using USB keyboards with fancy keys you are limited to maximum of 8 independent users ( for older versions Backstreet Ruby - 4 users). So do use hotplug if you are going to setup a system for more then 4 users ( even systems with 2 attached USB keyboards benefit from using hotplug).
If you are using hotplug the maximal number of independent users is 16 for Backstreet Ruby / Ruby-2.6 and 8 for older versions Backstreet Ruby.