I did the same thing a few weeks ago...I had an old GEFORCE 5200 FX video card with only one vga out. So, I got a new video card with 2 outs (Geforce 7600 gs only $120). My two monitors worked perfectly. Then a week later I realized I wanted a third monitor (lcds are cheap these days!) So, I thought I only needed a splitter...I was wrong! So, I got another video card--this time PCI because the AGP slot was taken when I upgraded to the 7600gs. To be safe I got the same brand video card from Nvidia Geforce...just a much lower model one to hook up the third screen, which was only $45. I installed that cards display drivers and all was fine.
Now, my triple display is sweet! I don't see how anyone can daytrade with less than 2 screens.
In sum, you need an extra video card--Or just upgrade to a video card with 2 video outs, which will be a DVI-out and a VGA-out (and they will come with a dvi adapter!)
Easy as pie...just open up your computer case and swap video cards...anyone can do it!