There was a similar question a couple of weeks back that you might want to check out. Lots of good advice! Here's what I wrote then, which I think may still apply to your house. It was for a 7 year old girl, but would definitely work for a boy too! Just switch the rewards to be boy-appropriate... :)
My baby is only 14 months, but I taught 1st and 2nd grade before I became a SAHM. And 7 year olds can ARGUE! It's enough to make you crazy!
Have a sit down talk, and explain the rules very clearly. (When you're not mad!) Pick a solid consequence for arguing, and then follow through every single time. Also, work together to come up with a solution for when she CAN tell you her differing opinions. If she understands that a different opinion is acceptable, but arguing is not, then you'll see a much happier and more mellow child emerge. Maybe she could write you a note?
I also used to ask my students, "Is this a 1, 2, or 3?" 1 meant not a big deal, 2 meant medium deal, and 3 meant HUGE deal. They only got one "3" a day, and only two "2's" a day. And "1's" were easy, we just laughed and moved on.
So maybe you could stop what you're doing and immediately listen to her when she picks "3." She can write you a note for her "2's" which you will respond to as soon as possible. And the "3's," she has to learn to let go of. And maybe you'll talk about them later, over dinner or something.
If this (or whatever you use) works, be sure to praise her a ton! And maybe even give a reward at the end of the week. Some are adamantly opposed to rewards, but I think they really work. Kids love to feel successful, and need to be praised and rewarded for making good choices! The best reward is time with you- maybe a lunch out together, or a pedicure, or something super fun like that?
Good luck! If it's any consolation, all those arguers I dealt with were always the smart ones...