I would strongly recommend AGAINST taking the RER into the city.
There are 2 different RERs you can take. One is an express and the other stops at every station through some of the most dangerous banlieue of Paris.
It is easy to get mixed up with the two due to the less than average RER information system at CDG. I personally know of one person who has been mugged on this line in broad daylight in front of a bunch of other passengers. If memory serves me correctly there is a shuttle from the new hotel into town, although I may be wrong.