The way I would go would be to replace your existing card with one that supports three outputs. Many of ATI's new cards have the "Eyefinity" feature, meaning they support three displays; but check the specs to be sure. They are surprisingly inexpensive. Here's one that can run three displays.
You can also keep the card you have and add an additional card, but that might not save you any money and will add complexity to your system. As Larry said, it depends on what slots you have available. If you have a free PCI Express slot, you can add a second PCI Express video card; that would be the least expensive. It's not that common to have two PCI Express x16 slots, so if you do have another PCIEx slot it will probably be x1. (You can use a PCIEx x16 card in an x1 slot, but an x1 card may be cheaper.) Here's a PCI Express x1 card that would work:
Zotac card on Newegg
If you don't, you can go with a PCI video card or a USB video adapter.