Which model do you have? What kind of monitor and res are you going to be playing at?
I have this model. http://www.newegg.com/Product/Product.aspx?Item=N82E16814130587
I had 2 of these in SLI, playing on an Apple 30 in monitor with 2560x1600 res.
If you plan on gaming on something with very high resolution, then I would say yes, get 2. If you don't plan on gaming that high, one is absolutely fine. I currently play BF3 on Ultra settings with 1920x1200 res on the 30...