using 3 or 4 monitor

just built system using NVIDIA GeForce 8800GT on 2 monitors and it
works fine...
However, when I added another video card to PCI slot (only one
available) it would only run 1 monitor.
How do I get to add a thrid (or fourth) monitor?

Thanks much...

Gigabyte EP35-DS3R
Intel Pentium 4 2.80GHz
4GB RAM
NVIDIA GeForce 8800GT
Broadcom 802.11n
Creative SB X-Fi
Vista Ultimate
Norton 360