are you looking to play games or spread applications accross all three monitors?
These are your four basic options given your system
1- http://www.newegg.com/Product/Product.aspx?Item=N82E168... this is the cheapest and least effective option. It should let you add a third monitor, but it will not be a good option at all really unless you want just a simple quick fix to browse the internet and such.
3- Any ATI 5000 or 6000 series video card, most likely something above a 5750. You will need either a monitor that takes displayport or buy an active displayport to dvi converter. Given the resolution of your first monitor, I wouldn't reccomend anything below a 5870 or a 6970 if you are gaming across all three, maybe even a 5970. With these you will definately need to improve the cooling in your system, as well as purchasing a new power supply because 300 watts will not be enough. I do not suggest nVidia because I beleive you need more than 1 card to hook up three monitors.
4- If it is for non gaming applications I would reccomend getting a workstation graphics card that can support 3 or more monitors. These can be quite pricey, but I do not know enough about these to comment.