I'm a mommy on the edge!

Every day is a constant up-hill battle with #1. I find that sometimes I expect way more out of him then I really should be. He is after all only 7 years old. He is a boy and isn't some unwritten law somewhere that "Boys will be Boys"? I understand that he is hyper and I get that all boy's make fart and car noises and other ridiculously annoying sounds. My question is when is enough enough, how much can I handle. I try to remain calm but often I loose it. Take this morning of example 30 minuets into my day and I was already yelling "ENOUGH!" Ugh that's a grate start to the day. Now have a good day at school, stay out of trouble.
Recognizing the problem is the first step. Now I must change, I must be the one to set the example. After all I'm the adult, right? I don't want to be a mean mom but I also don't want my kid to walk all over me. Think it's okay not listen and do whatever he wants to do. But there is a fine line and I can't seem to find the spot.
Time-outs, forget it he'd be there all day everyday. When I do send him there he gets out and does the same thing over again. "Hello uhm are you kidding me? You were just in time-out for doing that. What the...." Spanks, yup I believe in them, meh not so either. I mean if I say "Throw something at you sister again you get a spank" He wont throw anything else. Otherwise I will spank and low and behold, he does it again. The attitude and the talking back "Say it again you get your mouth washed out" 2 seconds later he says it. Reward system works for about a week, then he is over it.
I often wonder 'what did I do for him to act this way, where did I screw up?' Only 7 and I am already thinking/saying this to myself. Oh boy am I headed for long road.
Then there are day where is leaves going 'Who the heck is this kid?' He's helping out, he's getting things for his brother and sister, he's asking to do chores, he's respectful. Ah heaven. But its short lived. I must find out how to make him act that way all or at least 70% of the time.