The first thing that everyone's going to tell you is to save money by building your own computer, and they would be right, but I understand that not everyone has the time or patience to do that.
My advice to you is to buy the computer right before GW2 comes out, or 1-2 weeks after (if you want handle the wait). That way, you'll have a better idea of what the system requirements are and you'll be able to get a better computer for cheaper the longer you wait.

