I am looking at a Matrox 4-head card that is v1.3 OpenGL compatible: QID-P256PRO http://shopmatrox.com/usa/products/datasheet.asp?ID=445
This card is appealing because it provides 4 independant screens while comsuming only a single pci slot.
However, a search on this forum for Matrox topics revealed some unsettling reviews. I can forego an extra slot if the Matrox solution presents performance issues.
thoughts?suggestions?
If you are interested in 3D, then ATI or nVidia.
ATI has their 1300, 1600, 1800, 1900, etc
Some cards also have cross-fire support. Maybe you need the right mainboard.
nVidia has their Gf 6800, 7600, 7800, etc
They call it SLI. You can have 4 cards in SLI.
SLI/crossfire is “using multiple cards for rendering on the same screen”.
It doesn’t work across screens?
What if you create a window that is very wide so it covers 4 monitors?
Oh yes it works too, but you don’t need SLi/crossfire. Just use 2 dualhead cards.
That sounds cool to be sure. But why not just buy a bigger monitor instead, a wide screed LCD, for example? 
A good monitor will survive the next several generations of hardware. 
In all honesty I never understood the need for multimonitor display, with the possible exception of a panoramic flight sim, where the monitor borders could be imagined as the cockpit canopy supports (if you squint your eyes a bit). 
I have a unique project that requires multiple, independant video streams from a single application (media server): a club environment where video is distributed to multiple (8) projector screens.
Distribution consists of an 8x8 s-video switcher fed by multiple sources: one source being the Isadora application loaded on a media server streaming 4 channels of video.
The challenge has been finding a video card (video cards) that can stream multiple video channels (800x600) at a reasonable frame rate (>30fps).
Also, the video format needs to be S-Video (…sorry purists-distribution dictated this requirement!!)
Converting VGA to S-Video externally is possible but pricey – it would be a matter of convenience to simply pull S-Video straight from the card.
Matrox has a card that can do this, it is the QID Pro - the uncertainty being that this card is OpenGL 1.3 compliant and not PCI Express. I am not sure what to expect for performance…
note** the motherboard in the media server is the Asus P5N32-sli se deluxe.
So you’re sending multiple independent video streams to different monitors?
Well, I know bumpkis about this multimonitor stuff, so I had to look at a FAQ on it.
http://www.realtimesoft.com/multimon/faq.asp
Sadly, I’m none the wiser really.
However I can say with some confidence that there doesn’t seem to be a large demand for cutting edge, high performance video cards with 4 monitor outs, and perhaps understandably so.