Multi-headed Linux Displays

The following are the various options that we currently know about for getting multiple graphics heads (that is, 2 or more video channels with 3D acceleration) on a single Linux PC.

Evans & Sutherland Tornado 3000

requires the Xi Graphics LGD X server
2 video outputs; can be configured up to 1024x768 each
costs approximately $1300

Matrox G400 / G450 Dual Head

requires the Xi Graphics LGD X server
(also supported by XFree86 4.0's DRI (requires 2.4 kernel), but I don't know if that supports dual channels)
2 video outputs
cost ~ $130

nVidia GeForce 2 MX

2 video outputs
this is a chipset, used in cards such as the ASUS V7100 and Inno3D Tornado; NOTE: not all GeForce 2 MX cards actually implement the dual-output feature
the Inno3D Tornado is available in PCI as well as AGP form
requires XFree86 4.0 + the 0.9-6 driver downloadable from nVidia's website
cost ~ $120

nVidia Quadro 2 MXR

2 video outputs
used in Elsa Synergy III card
requires XFree86 4.0 + the 0.9-6 driver downloadable from nVidia's website
cost ~ $500

multiple nVidia-based cards

the latest (0.9-6) nVidia drivers are reported to support multiple cards in a single machine
this includes all recent nVidia chips - TNT, GeForce, Quadro
a system could use 1 AGP and 1 or more PCI cards, or simply multiple PCI cards
(I have had success with up to 3 PCI cards, but not with AGP+PCI; see my nVidia multi-head benchmarking notes)
cost: from ~ $60 (for two used TNT2s) on up

Future Possibilities

The following are cards and software that have been announced, but are not yet available, as far as we know.

ATI Radeon VE

will have "Hydravision" multi-head feature
the current Radeon is supported by XFree86 4.0's DRI and by Xi Graphics' LGD X server

3DLabs Wildcat 5110

announcement says it will be multi-headed
3DLabs claim to be working on Linux support for Wildcat cards, but have not released anything yet

Xi Graphics LGD version 2.0

This page says that the next release of the 3D accelerated server from Xi will have "DualView support" and "Multi-head support". Presumably "Multi-head" means multiple cards, but the short description is unclear.
(It also says that they will provide support for active stereo.)