Actually your concerns are very valid. You will need to do some homework before getting another dog, mostly in assessing your dogs temprament and daily routine then assess the new comer the same way.
Does your dog get daily walks? On a leash?
How well is your dog trained?
How does your dog spend its day?
How often does your dog meet other dogs?
**How does that work out? is your dog okay with other dogs or is it agressive or cal, or does your dog flip over and show the other dog its belly??
How good is your dog around its resources? -food, toys etc.
Do you feel your dog would guard its food? Its toys?
How spolied is your dog?
Allowed on the couch? On the bed? etc.
How much time do you spend with your dog?
How much time do others spend with your dog?
Does your dog mind you? Right away or at its own time?
Does your dog "dry hump" anything - this is an act of assertiveness - has nothing to do with sex, which is why I asked.
Next
Do you have the experience to determine if a dog is agressive or assertive?
Can you take your dog to a dog park?
Is it possible for you to meet a prospective dog without your dog? My suggestion is that you meet the dog, take it for a couple of leash walks, do a bit of training with it, determine how well it obeys etc. When you take your dog to mee the other dog, do it on neutral territory and take the dogs for a leash walk (this establishes you as the leader)
Let them get acquainted. Determine which dog is the most assertive - you will need to establish yourself as the boss right off, and working with a more assertive dog without much in the way of behavioral modification will be harder on everyone. You will need to supervise play and behavior while they are eating or around your current dogs established areas - like a bed your dog might be aggressive around, guarding behaviors will start with body language and escallate from there and could result in a fight. The good part about all this is that you have a good breed for temprament. Make sure you get the dog from a reputable breeder or a rescue group has worked with whatever job you choose.
Good luck, and feel free to share with us your decision and the photo of the new addition, if it becomes a new addition.

Mazar,
thanks for bringing up a lot of good points. As good a dog that we think he is, after reading some of the questions you've posted I think it might be best to wait.
Thanks for the help.
Kim