The magical marble jars

We have had a few behavior problems here lately in the house, particularly with Sam, our pronounced MIDDLE child. I am hoping that when he shares that role with Olivia in about 3 months, he'll get over it, but I won't hold my breath!
I discussed the situation with our pediatrician who recommended a jar with marbles, where the kids earn marbles for good behavior, and lose them for bad. It is very visual and they can redeem them for rewards. We've done it for two days, and it is AWESOME! It managed to take care of two problems at once. The boys got a Wii from their Grandpa for Christmas, but since he was in for Thanksgiving (from Alaska) and won't be back for Christmas, they received it last week. They could basically forgo food and sleep and play Mario Kart All.Day.Long. if we let them. Asking them to take a break results in an eruption of whining and crying, and gnashing of teeth. Plus, I'm a weenie and it IS kinda nice to be uninterrupted in the afternoons while they zone out.
We actually convinced Sam the other day when he had a nosebleed that his brain had turned to mush from playing too many video games, and was leaking out of his head. We told him he needed to take a break before it ALL leaked out and he couldn't think anymore, and that if he didn't play for a while, it would have time to harden back up. I asked him what day it was and when he didn't know (which he never does if it's not a Tuesday or Thursday when they talk about it at school!) I gave him the old "OH NO! That part must be the part that already leaked out!" - Yeah, we're totally prepared to pay for therapy, let's just hope not playing too many video games makes up for it with a scholarship ;-)
ANYHOO, I digress. The marble jar. The kids earn marbles by playing well together, staying their seat at mealtimes, sharing, using good manners, being good at school, etc. etc.
When they accrue 10, they can redeem them for ONE hour of Mario kart, during which time they cannot earn marbles, but can still lose them for bad behavior. They can earn one if they are good when it is time to turn OFF the game.
We love the marble jar, oh sweet peacemaker among siblings. You, marble jar, are my new best friend. We'll see how long this friendship lasts.