This RAM will work for the XPS or the 9300, and yes you can mix it with the RAM already installed by Dell although it is always best to have a matching pair.
I ordered my RAM from CIConcepts today and spoke with their sales guy, Kevin. He said the RAM will be shipping from Kingston on Wednesday. He seemed very nice and for 131 bucks its a nice deal. Shipping there is kind of costly however, 10 bucks for ground. Other than that they seemed like great people.
I purchased from there. I will report back once I receive the RAM. Happy purchasing all.