If memory serves, more video cards won't be your problem, but the way windows handles the resources and it hates multi-monitor setups for some reason. Most of this can be overcome with having the right drivers installed but ever since the Vista days and the Aero desktop, multi-monitor support has been crap. Advances in hardarware has overcome a lot of this by simply brute forcing past the bottlenecks, but it's still there as far as I know.

My work machine runs win7 and has a 7900GTX in it. It will play HD videos just fine on the primary monitor, but if you slide the video over to the second monitor, it just chugs. Even today there are lot of games that simply will not let you run them in full screen on a second monitor.