This is the exact age range of my two - also opposite sex (only my boy is the oldest and their birthdays are 3 years and 2 days apart). Everyone says 3 years is the perfect age gap, but I found it to be pretty difficult in some ways. Of course, having a girl for the oldest might change this, but my boy was still quite immature at 3. He struggled to complete potty training - last in his daycare class. And I personally found it hard to be pulled in such different directions caring for a newborn and a 3 year old whose needs were very different. I don't say that to scare you, but I sort of wish some one had tried to prepare ME a little bit - not just my son.
Beyond that, I think there are several good things you can do to help your daughter adjust. There is a Sesame Street video called Three Bears and a New Baby. It's a good one to watch. There are some good books about becoming a big sibling. Our hospital had a big sibling class for us all to attend.
We moved our son from the crib to the toddler bed well in advance of the baby coming so hopefully he would not hold her responsible for him being kicked out of his bed. Someone told me that we should call the baby "his" baby. By giving our son ownership, it helps foster a bond and a special feeling in being big sibling.
We bought gifts to take to the hospital - one for our son to give to the baby and one for the baby to give to him. They were wrapped up. When DS came to meet DD in the hospital, they exchanged gifts. Also, we made sure that I was not holding the baby at the moment DS came into the room. I did not want my arms to be "full" when he needed a hug that first time.
Someone else told me that it is helpful to the older sib if they hear you tell the baby to wait sometimes. Big sib will be told so many "no's" "Don'ts" "I can't right now because of baby," and "wait a minute's" that its hard NOT to be jealous. But sometimes if you just say outloud so that DD can hear you, "New Baby, hold on just one minute, I'm getting Big Sister some juice." This helps Big Sib feel some equality. Yes she will have to wait a lot, but it helps to know that Baby Sib has to wait sometimes, too.
Best.
J.