Simply put, it's not about the graphics card so much as the software.
Get the best card you can buy from either nV or ATi, and then use UltraMon or some other multimonitor software dedicated to doing it well. Neither Ati's Hydravision nor nV's nView do multi-monitor as well as Matrox, and the only thing that will give you the same experience IMO is using a 3rd party program like UltraMon.