Getting another dog is almost always a bad idea. First off there is no guarantee they will get along with one another. My girlfriend recently moved in with me and brought along her female puggle "lucy". I already have a male Jack Russell "coltrane" and although they get along well for the most part, its a lot more work having two dogs.
Most people think that getting another dog will free them up because the dogs will play with one another and need less direction from you. This is not usually the case. Two dogs get into a whole lot more trouble than one solitary dog who is content sleeping his 8 hours a day.
If,however you must get another dog, make sure its of the opposite sex of your current dog. Same sex aggression is quite common and everything from chew toys, your affection, and kibble may become a competition for the two dogs to see who is the leader. The poster who mentioned that dogs are pack animals is correct, however most humans are not capable of leading a pack of dogs. 3 dogs that live together and form a pack without a dominant leader being the human is a recipe for disaster, here is a good article on what can happen when dogs become a "pack"
http://www.leerburg.com/rot-kill.htm
We have spent a lot of time and energy training our two dogs (at least I have) and I can honestly say that it would have been so much easier if we just had the one dog. The poster who mentioned the roommates noise bothering you brought up a great point. I bet on sports for a living and am now building a sound proof media room because the noise these two dogs make while playing or barking at squirrels or playing chase can be extremely distracting.