Which aperture is best for portrait photography?

I've been taking pictures of couples/families, etc and I'm having trouble with which f/stop to use. Sometimes, both people are at the same distance from the camera (they are next to each other), but the camera seems to focus on only one person. DOF is supposed to be in distance from front to back, right? And not side-to-side? I ussualy shoot on Aperture Priority Mode and my camera is a Nikon D40, with a Nikkor 18-200mm lense. It seems as if I need to use at least f6 or more (f8, 9...) for both people to be in focus. However, this does not always blur the background as much as I would want. Could this be because the D40 only has 3 focus areas? I asked a professional photographer and she told me f4 would be fine... but once on the computer, I see that someone is out of focus... Also, when shooting two or more people, where on the subject should I look the focus area? Any help would be good! Thanks!