Even though we new we were having a girl, we went sort of neutral sex-wise, with primary colors. The walls are a medium-intensity yellow (called "Lucy" at True Value Hardware) and we found a border at Lowe's with blue, red, yellow and a soft green flower design. We used it at chair-rail height, since the room has 11-foot ceilings.
When baby #2 came along, and was a boy, the primary colors kind of outshone the fact that there were flowers on the wall.
Both kids love the movie "Cars" and Lowe's has these things called Wall Pops - think removeable wallpaper art - and out went the flower border for a stripe, with the residents of Radiator Springs smiling from the wall now!
Just recently, my daughter, who is 5, decided she wants her own room, so we need to decorate the guest room for her, and we'll go back for more Wall Pops. They have dots and boxes and all kinds of fun things. Meanwhile son, age 3, is still enjoying his Cars. I think Wall Pops rock!
Good luck and don't stress over the decor. Have fun, and remember, a baby of one sex isn't going to be offended if his/her room isn't "right." Just wait until later - I've come home from work to find my daughter has dressed my son in her Halloween witch's skirt and pink ballet shoes! I just shake my head...
(Lately he has drawn the line at wearing her tutu from last year's recital.)