I was just looking up whether Eyefinity (by AMD/ATI) would be a good alternative, so I'm not sure whether Nvidia's cards enable you to do so. Though like BBlake said, I don't think Windows will limit you.
The AMD site notes:
Number of Monitors:
• Up to six dependent on graphics card configuration
Aggregate Screen Resolution:
• The aggregrate of screen resolution will vary based on the number of panels and screen
resolution. The theoretical maximum: 8192 x 8192 (67.1 megapixel resolution) consisting of 6 panels of 2560 x 1600
• AMD recommends DisplayPort Display monitors, as they offer optimal flexibility with the highest number of display outputs
• Depending on the specific graphic card and operating system, ATI Eyefinity technology can
support up to six displays. For configurations using more than two displays, additional displays must have DisplayPort connectors.