I'm sure she is feeling "neglected" because of the new baby EVEN THOUGH I'm sure you are NOT neglecting her. It is totally normal. I have two kids close together so I know exactly how this goes and the more you punish the worse it gets; however, that goes without saying that she DOES need limits. Have you ever heard of or read Parenting with Love and Logic? It is great and a very easy and quick read...get the one for under 6. Also try to fit the punishment to the crime if that makes sense. And try to not show frustration...that is all what Love and Logic is about...it is SUPER hard at first but you'll get the hang of it real soon. If you have anyone who can watch the baby for even an hour or two..try taking your daughter out for some along "mommy time"...you'll be suprised how different she probably acts away from the baby. As you probably know she wants to be like MOMMY...so try and ask her for lots of help with the baby and model exactly how you want her to behave...helpful, good manners (please and thank you)...after a couple days she might get the idea.
Also, have you tried a behavior chart? My daughter really responded to that b/c she could earn rewards. It isn't for every child but if she likes getting "checks" or "stickers" it might work. We made our own...I put like 6-8 things on the side and then across the top was the days of the week. They were things like sleeping all night in her room, brushing her teeth, pick up toys, take nap/quiet time, etc...decide how you want to do it and then at teh end of the week she gets a reward...a new book or doll.
Hope this helps! It will get better!