Well, I have a 4.5 yo daughter and an 8 week old. No major adjustment or jealousy issues. Actually, I am so impressed by how well she has adjusted thus far. This is what I did to prepare her:
1. Dont let the baby equate to a "loss" or that it is taking something away from her. From the very beginning of my pregnancy, I made a point to never blame the pregnancy or the baby for any negative response or consequence. If she wanted to do something and I was feeling sick, tired, unable to sit on the floor, etc.,--I never told her I couldn't do it because of the baby.
When her sister was born, again, never told her she can't do something or have something because of the baby. If she was invited to a birthday party 2 weeks after delivery, she was there. School parties and dance recitals, I was there... Granted I had the baby in a stroller (no touching rule) and I took my nursing pillow (I have 2, one stays in the car) and was ready to nurse if necessary... But whatever the level of effort I made pre-baby should be the same effort and routine post-baby.
2. Celebrate her being a big sister. Let her pick out things for the new baby. I told her that part of being a bug sister was to help buy the car seat--we bought the one she liked. When buying a stroller, the model came in 3 color choices--she choose the color. She choose the 'going home from the hospital' clothes, etc.
3. Any stuffed animal that was given to the baby (from a shower) was hers. It's not worth fighting over (we only got a few anyway). The baby is too young to care.
4. She moved from her room to a new room. Her old room was refered to as the 'baby room'; her new room was called a 'big sister room'... And yes, we redecorated her new room.
You mentioned that your daughter was going to lose her playroom. Instead of turning the playroom into the baby's room, why not move her into it and give her a brand new 'big sister' room. Then use her old room for the baby room? That way, she gets promoted to a new room, new decorations/bedding, etc. (Of course, let her decide if she wants a new room)
5. She claimed all the stuffed animals and toys that were in her old room... All of them, even the ones thatwere sitting way up on top of a bookcase as decoration. Fine, it all got moved into her room because again, it's all about giving her a good, positive first impression of the baby. She needs to feel she's not losing anything. Now, granted she has so many stuffed animals in here that they overflow out of her toy box; toy kitchen and doll furniture is in her room too. It is crazy crowded. But tell yourself, it's all temporary; it's only for a few months and then Christmas/birthdays come... I told her, "if you want new toys for Christmas (or her birthday), you need to give some up, because you don't have room for any new ones." Hence, her impression is that the need to give up toys is because new ones are coming, not because the baby is taking them from her.
6. Lastly, I'm throwing my daughter a big sister party to celebrate her becoming a big sister. (Actually, it's tomorrow!) Yes, it feels like a birthday party (invitations said present were not expected) but it takes the focus off the baby and puts it back on her. I'm planning on toasting her, recognize how helpful and selfless she has been, recognize that she's been such a good big sister to share toys, blankets and mommy.
Good luck with everything!