I used bagged spinach for this ... I only used half the bag and thought it was easily enough spinach for 4 cups of pasta.
I also used whole wheat rigatoni (I was out of penne), and any pasta works fine!
When blending, I just added the red pepper into that, as I don't always prefer big chunks of stuff in my sauce. It'd be a great way to "hide" veggies from kids!
Mine turned out really green, but still tasted great! I will definitely try this recipe again and maybe add some grilled chicken to it.