After removing the wallpaper from my living room, I find bare plywood walls. What should I do before applying mud and texturing the walls?

I have a pretty small house, and I wanted to open it up a bit. I decided to cut a "window" in the wall between my kitchen and living room. Before starting, I had a carpenter friend of mine come over and look at the project. He informed me that the wall was load-bearing and therefore I would have to keep some type of supports in place. However, since I started the project, I've started talking to other people who say that since I have a one story house, the only thing the wall is supporting is the plaster (in both the kitchen and the living room) and the type of support I need isn't very much. This leaves me with two questions: 1) The size of the hole I'm cutting (roughly 60 inches across) requires that I cut through 3 vertical wall studs. For support, while working, I was going to nail a 2" by 6" across those three studs, plus one more on each side. I was then going to cut below this 2" by 6" and frame in the "window". Is this enough support for the ceiling, or will I run into trouble? 2) I'm a bit confused as to what type of header I'm going to need. So far, if I'm thinking correctly, I'm going to have to add one or two 2" by 4" to each side of the hole (vertically, nailed to the first uncut stud on each side) and then have a header that will rest on these, going from one side to the other. I am then supposed to toe nail the three cut studs to this header. My question is, what should I use as my header? I've had some people suggest a 2" by 6", or two 2" by 6" nailed together. How should I proceed? Thanks for any advice.