It really depends on what you're doing. Any video card with the proper ports will run the monitors, but if you want to do graphics intensive tasks at high resolutions you'll need something beefier.

The basics are that PCI express x1 and x16 are different "sockets" based on the same technology. In a technical sense x16 is much higher through-put and great for higher-end graphics where they need the bandwidth. But for your purposes it's probably going to come down to the what ports you have available.

Since your system is not SLI enabled you can only have one PCI-Express so you'd either need to replace it with a card supporting 3 or 4 DVIs or get a x1 as a secondary card.

It can be a little tricky to get multiple video cards to work right if they're not in SLI, so I'd suggest something like http://www.newegg.com/Product/Produc...82E16814121247 to replace your current card so you'll be covered no matter what.

I have a dual monitor set up. I have a 512 ATI card with dual DVI. I wonder if you could use 2 of those.

Not in this case, since that computer only has one PCI E slot, but it can work on a system that supports two PCI E cards.