The easiest route is to buy two cheap $50 video cards. ATI vs Nvidia doesn't matter, since all you care about are the display outputs.
If you want a single video card, you'll need an ATI 5000 series, but you'll also require output adapters since . Tom's did a review of the 5450, using a 3 monitor setup. You'll also need to grab an "active" DisplayPort converter, which today can be found for under $25 if you know where to look.http://www.tomshardware.com/reviews/radeon-hd-5450,2549-3.html
No Nvidia cards support 3 displays from a single card, you have to go with multiple cards for 3+ displays