assuming you aren't using the card for gaming or 3d rendering, etc you could go with 2 AMD cards- they should be able to power 3 displays each (2 monitors will use DVI output and one will use Displayport for each card). You will need to make sure that your motherboard is Crossfire compatible (this is the term for using two AMD graphics cards together) and that your Power Supply candle handle the power requirements.
The HD 5770 x 2 would be plenty of power and can be had for around $100 for each card. You can definitely go lower end if that's more than you want to spend.
Beltzy, unfortunately you can't do that with ATI cards.
ATI (AMD) cards in crossfire still have ALL of the display outputs go through the 1st card.
@Ronstar111, If you intend to use 5-6 monitors on one card, the best solution would be to get an ATI Eyefinity 6 edition card, which will have 6 displayport adapters. Be prepared to buy a lot of active displayport adapters too, as I'm sure the monitors themselves won't have DP ports on them.