First of all, what desktop? It has to be able to support the new cards. For example, the slot the card is in could be PCI, PCIe, or AGP.
The easiest solution would be to add a couple cheap cards and run two monitors off of each, such as a couple 4350s.
My computer is a Compaq Presario E51DEBCB (5008 US) with XP Home Edition service pack 4. The processor is 800 megahertz Intel Celeron. The lnternal built-in display card is Intel(R) 82810E Graphics Controller
I have one NVIDIA PCI GeFORCE FX 5500 OC Graphics Card 256MB DDR running one monitor. I have 4 total slots available for graphics cards. Can I run 4 monitors with four identical cards in the 4 slots? I build webpages and I'm not a gamer. Thank you.