OK. You can go retail and get any Nvidia PCI card, or you can buy a new Matrox G450 or G550 PCI card... about $100-$120 new or $40-$50 used on eBay. Both of the Matrox cards will enable you to add 2 more monitors.
http://www.matrox.com/mga/products/mill_g450/home.cfm Takes 2 minutes to install, make sure you are grounded!
The G450 has 2 ports. The G550 has a dongle/splitter. It will come with the card if purchased new, but you might have to buy it separately if you buy used off of eBay.