Yes you can run 6 monitors on 3 PCI cards, plus 2 more on the PCIe slot quite easily.
The only question becomes what you want to do with them next.
For desktop use, no problem; for video playback a bit tougher; for gaming tougher still.
For some things you may want to get a good dedicated multi-monitor application, like UltraMon, to make management easier/better.
You could also look into someone like Matrox for their expertise in multi-monitor support. Then you could do 8 monitors on 2 cards. Matrox RULZ !!