I looked at some Arctic CPU coolers and people say they are no good for AMD CPUs. I found this one though:
http://www.newegg.com/Product/Product.aspx?Item=N82E16835103099
People seem to like it. It also seems to come with thermal paste so I don't think I need to buy any more do I? And I can put this thermal paste on even after applying the stock thermal paste right?
One last thing, does it matter cooling wise if I leave my case open or closed?
I have an arctic and its spot on, maybe I'm in a minority. Also, I maye be wrong but I've heard Coolermaster heat sinks are pretty loud. It's something you should consider when buying a CPU fan.
They do come with paste on yes, but it's usually standard stuff that isn't very good quality. From my research, arctic silver paste is the favoured product for overclockers. If you put your own on, I'd suggest buying the removal kit to take it off the heat sink before applying your own.
Also look on YouTube at how to apply the paste, there's a specific method to use.
I can't speak for an open case, I suppose it would help with heat, I've never ran my pc with an open case before.