A.,
I have a 9 year old and 3 year old boy. Things had recenlty gotten out of control, sounding very similar to your situation. My husband and I put the hammer down.
For the three year old, we give him one warning (unless it's a continues reminder for us) then we say "oh how sad" and take him to his room. We tell him he can join us when he's ready to be sweet, follow rules, etc. (Simple is the best, and the only emotion you show is the one you have from them having to spend some alone time.) Very similar to what Rebecca said.
Our 9 year old, we do the same thing. When it becomes a repetitive thing or a little larget we take privilages away.(ones closely retated to the problem/favorite) We'll even ask him to come up with some ideas. Amazingly, he comes up with some fair ideas (doesn't mean you have to use them). I also have to run into having to remind him to finish/do his chores properly or to even do them. After a couple of reminders (when I'm ready to burst) I'll either do one of two things. One, suggest that he restore my engergy by doing one of my chores. The second, is to just do it for hime and he needs to store my energy.
When he restores my energy...I find myself less stressed and frustrated. It's like it never happend. Now when he chooses not to restore my engery, I wait until there's something he wants me to do for him, like take him to his friends, make his lunch, etc. Then I tell him (in the somes sincer voice), I don't have any energy left. Whould you like to restore it?
I cann't begin to tell you how great things have been in the house since we've started this. These are both methods from "Love and Logic". You may want to check out there web site. They offer great tips and sell books, audios,ect. Some counuities even offer courses.
Good luck!