Switch out your ram for a dual channel kit like this one - http://www.newegg.com/Product/Product.aspx?Item=N82E16820220555
- not only is it cheaper with a rebate, it is dual channel. If you don't like rebates, there are others to choose from at the same price as the one you linked.
I also agree that you might want a 6670 and one of the PSU's that zooted suggested.
You don't need to upgrade your case unless you need special features on the front of it (like USB 3.0, eSata, and so on) or you are buying an odd sized mobo (which you aren't). I'm using a ~20 year old case for an Intel Atom server I built a few months ago.