Hi J.,
Congratulatios on the new addition to your life.
I want to echo what the other posters have said. Your 5 year old needs mommy time and attention. Of course you can't ignore your baby, but try to give our older son special time. And try to include the older one with things you do with the baby. If I saw my son acting out I would ask him what was going on. If there was no reason for him to be acting out I would ask him -- are you trying to get Mommy's attention? If he said yes, I would remind him that he just needs to tell me when he needs Mommy's attention and I will get to him as soon as I could. It didn't take him long for him to tell me, "Mommy, I need some 'tention." I would finish with the baby, or just stop what I was doing (if not with the baby) and just listen to him tell me whatever he wanted to say, or do whatever he wanted me to do if I could.
My children are not quite 3 years apart (boy then girl). I used to bathe them together for the first year or so; then I could play with both of them. I would read to them at bed time -- both at the same time. If my daughter went to sleep first then I would read my son another book while he laid in bed.
If my husband was not working late I would have him spend time with my son -- or daughter, depending on what was going on -- while I tended to the other. Special time with Daddy is important too.
Once a week my son and I would have "special time" with just us. That could be going to the playground, movies, taking a walk, etc. His world is rocked and he wants to make sure there's still a place and time for him in your life. Words won't always assure him, so actions need to speak. You don't need to put up with wrong behavior, just understand where it's coming from, help him to articulate what he needs and is feeling, and make him feel heard.
I also suggest the book "Siblings Without Rivalry." While there really is no such thing, the book helps with reading underlying issues but also with any guilt you may have about giving one child "more" when it is actually what the one child needs at that time. The book will tell you that you need to give each child what they need at that point, and how to explain to the other child.
Trust me, all of this goes a long way with siblings -- especially into the teen years (where I am now - 17 and 15).
Best of luck to you.
S.