Combining Econ with CS, particularly if you intend to focus on stats/data mining/machine-learning will be a really strong combination. I did the professional masters program in CS at the University of Washington, which was a ton of fun.
You'd likely have to prove that you know the basics of data structures and algorithms and have at least one programming language down pretty well. They might want you to take another class or two at the undergrad level. But, if you have a compelling goal in mind, programs will probably be flexible.
