Im not 100% sure you can use a dedicated video card and have the integrated graphics on at the same time. (Im thinking it could vary depending on which motherboard) Even if you could enable them at the same time you would need two sets of drivers to run them, which is only supported by windows 7 currently.
If you cant get it to work with only one card + igp you could get two cards since you have two pcie slots on your motherboard.
For a third option I should also note that the ATI 5000 series supports up to three displays, at a cost. The 3rd monitor that comes out of it has to use the DisplayPort output and you will most likely need to use a DP to DVI/HDMI active adapter, $100 a pop, no cutting corners with passive adapters. You can use a DP to VGA adapter, which is alot cheaper than the DVI one.http://www.newegg.com/Product/Product.aspx?Item=N82E16812270279
This DP to VGA adapter is no longer sold at newegg but it works in my setup.
For options on graphics cards, anything cheap: 5450, 4550, 4350 will get the job done.