Quote from tnc$ddc:
Not knowing what brand or how many graphics cards you have, it looks to me like you may need at least one more dual graphics card to add 2 more monitors.
You need to make sure the card(s) match the type of slot(s) you plan to use them in (AGP, PCI, PCIE, etc.).
When first setting up my trading computer, I attempted to mix different manufacturers graphic cards, and had serious problems. I know others have done it with great success.
I eventually replaced all of my cards with Matrox cards. I presently have one AGP (P650-dual) and two PCI (G450MMS-Quad) cards. I've been using this setup, with 8 monitors, for over 3 years, with absolutely no problems.
When I bought my cards from Matrox, if I remember correctly, the P650 was $169.00 and G450MMS cards were around $700.00 each.
You may be able to find cheaper cards elsewhere.
Don't forget to add the price of a monitor stand, if you need one.
Hope this helps.
TC