I'm assuming that you have an available PCI slot in addition to the PCIe. Just place a dual-head card in the PCIe and then another video card in the PCI slot.
Matrox sells a 4 in 1 PCI video card, but the above solution would better if you need higher end graphics for other than trading (games, video, etc).
Next year, a new display connector standard will come out called Displayport.
This will allow up to 4 outputs on one card. Of course, you'll have to have monitors (none available yet) which support this standard.