Have you considered stenciling a pattern on the existing walls instead of wall paper? I saw this on a show once and it came out really nice. It is a lot of work depending on how close together you stencil the pattern, but it had a very interesting look. If I remember correctly they made their own stencil and it was a kind of abstract shape rather than a flower, etc. It was stenciled randomly all over the wall in a couple of different colors. It really jazzed it up and it may be a whole lot easier than trying to wallpaper it. Another idea might be to try the ragging technique, which would add interest to a textured wall. Hope this is helpful.