It sounds to me like she is in desperate need of attention. Maybe she is feeling left out? Perhaps some one-on-one time with her during the day will help. Maybe spend an hour painting, drawing, cut and pasting or take her to the park.
Kids usually act up when they want attention - any kind of attention is better than none. When i was little I was such a terror, my mother told me i used to tell her i liked to make her mad. I remember acting up as a child and i clearly remember it was because I felt like I wasnt getting enough positive attention.
The reward system is great, but threatening to "take her to hospital" isn't such a great idea. I'm no psychologist but it's not a great thing to encourage a fear of hospitals/dentists/doctors etc...
Smacking only encourages violence and make sure you don't yell at her or call her a "naughty girl". Encourage positive behaviour.
I have always found self-education to be the best solution, so perhaps go and have a look at your local book store for some decent literature on child behaviour.

Very best of luck. I know how frustrating it can be, but take some time to try to understand her and why she is behaving the way she is.
Sorry, but i didn't come in here to be reviewed at how i parent. I don't appreciate at being told what not to do. Especially when told i am encouraging violance in my children. Thanks!

Trillian, yes she actually does eat a lot of fruit. She doesn't eat much else during the day. I even have to put banana on her sandwish. She has always been a fussy eater and if it's not fruit she just will not eat it. So i'm not sure what to do there, i would rather her be eating.

She doesn't go to preschool anymore(just got way too expensive) but most of the day is spent reading books(she loves this), playing with toys, drawing and a bit of fun outside. We had to stop painting and playdoh because DS would just eat it and i didn't think it was fair that DD could and he couldn't play with it. Maybe after christmas we could try it again. But like i said, she is craziest at night after dinner. DS is already in bed. It's like the sun goes dwona nd she turns into a beast lol.

Today they both have colds, so i reckon she won't be putting up much of a fight tonight. Poor thing just wants cuddles.