I remember feeling exactly the same, Spritesoright, when I was pregnant for the second time, and worried about it a lot. How on earth could I possibly love another child as much as I love my first? But of course, love is limitless and you do love all your children equally. Just try, when you can, to make sure you involve your first child in the care of your new baby.
When feeding your newbie, read a story to the older child at the same time, making sure he/she has a drink and snack and you're all settled down together. When doing something as simple as changing a nappy, maybe get your older child to help somehow, like get the wipes, clean nappy, lots of praise etc, then he/she will feel helpful. The older child will only "act up" if they feel neglected. It may be hard at first, to make sure you concentrate on both or all of the children, but it will become part of the routine. And when new baby is asleep, set aside some time to do an activity with your toddler, a jigsaw maybe, or just playing with construction toys. Even if you feel the need to do some cleaning, just wrap her feet in dusters and get her to skate up and down the hall. That way you're keeping her entertained, she feels important, is having fun, and you're getting some housework done too!!! (You probably know all this already)
I can honestly say I never had any problems with jealousy or resentment with my daughter towards her new brother.
Please don't feel afraid that you won't love your new baby as much as your toddler, you undoubtedly will. Personally, my feelings about my first child did not change when the second arrived.
My children are now teenagers and young adults - NOW they moan about who is loved most
(Depending on who thinks the other has had more money spent on them) Sigh