I recently bought the external graphics card TripleHead2go by Matrox.In essence you plug 3 monitors into it and use the fourth port for another screen. I soon found out it does not work with older ATI cards or ATI Crossfire. Once again NVidia's are the recommended choice.
If you are in the business for the Matrox take a look at this compatibility list.
In the end I settled for a later ATI model. The 4350 -an entry level card- which has a stronger GPU than previous entrly level models and hopefully lower power consumption.
The passive cooling on these cards is very bad and I recommend to change it to an aftermarked solution like Accelero.
Working with four screens is great. Imagine you have to monitor weather conditions or you have to maintenance some machines.
Expanding to eight screens with the usage of the second DVI port of the graphics card and another Matrox TripleHead2go would be possible in theory. Eight Screens - Total overkill.