It wasn't potato chips, it was nuts (nutz). And what Jimmy says in the commercial, essentially, is "You're not nuts." That is, he seems to look right at her and tell her that she's not crazy for thinking that Don had an affair.

She was wavering, thinking that if there's no evidence maybe she was "nuts." This commercial strengthened her resolve that she wasn't crazy--and reminded her as well what Jimmy had said and shown her (the way Don and Bobbi looked at each other). Likewise, the commercial tells her that if she believes it, she'd be "nuts" to just accept Don back as if nothing happened.

I think Jimmy's defining moment was when he pointed out Don and Bobbie at the bar together. It was obvious that the two of them had something going on. Up to that point, Betty had only suspected the affairs. Seeing Jimmy on TV brought home the realization that Betty can't go on pretending. Although I disliked Jimmy for involving her, he may have done her a very big favor after all. At the tail end end of that commercial, an animated character winked, which may have sent her a message as well.
Theory #2: Betty had already been subjected to a bit of humiliation at Jimmy's hands (and thanks to Idiot Don) at the restaurant...where she was genuinely doing Don a good deed. Then she has to endure even more embarrassment at The Stork Club as Jimmy blows Don's cover. I think she is sick and tired of being used all over the map. She feels she is due way more respect and it's high time she took a stand. The TV commercial just lit up all the fireworks.
Theory # 3: I agree with you, Thirteen!

And I agree with you Jolie! Once again, she'd be "nutz" to let Don humiliate her again, right?

Yes, Thirteen. She had also stated that Don embarrassed her at the dinner party by not jumping in right away to explain the Heineken beer "joke". He just stood there while everyone chuckled at Don's wife who bought the imported beer,
which proved that Donnie Boy knows how to market a product. Har har har.

Hi fellow Maddicts! I think that seeing Jimmy, either on TV or in person, after her last encounter with him was just too much for Betty. The Nutz connection was brilliant, Jimmy saying he isn't nuts, Betty wondering if he is, and Betty going nuts herself.

I think that when she saw this nasty, despicable little man on television, stuffing his mouth with nuts and potato chips and grimacing for the camera, Betty had to feel disgust. Disgust for Jimmy, Disgust for Bobbi and certainly Disgust for Don.

I've also been thinking about what was going on in Betty's head while she watched that commercial, and agree that it's about her total realization as to how Don plays her all the time - that she's not so much his wife, as some kind of marketing test and image enhancer. I really felt a gut punch when she's standing there trying to figure out the laughing reaction to her Heineken choice. She had spent so much time and effort to come up with a wonderful menu, and to be a gracious, classy supportive hostess, and her husband thoughtlessly collaborates with an outsider "against" her. Again. Remember how happy she was on the ride home from Lutece, having played the part Don asked for to save the Utz account? While she watches Jimmy's commercial, she realizes that while she was doing her thing in Lutece to support Don, Don and Bobbie were doing their thing. Everyone has their tipping point, and the Utz commercial just brings her past it. She believes Jimmy, doesn't believe Don, and she can't take it anymore. Good for her, because it was long time coming.

I think the Jimmy ad was the trigger for a combination of a lot of these reasons. Good job, guys! Indeed, my theory has always been that half the reason that Draper slept with Bobbie was for business, and seeing that ad reinforced that for Betty. Suddenly, she had a motivation for what Don did, and more than a gut feeling. And going through his stuff, note that all she finds is "stupid advertising on cocktail napkins." She's sick of how Don is all about business, 100% of the time. He's like Howard Roarck in The Fountainhead, all about work.

The funny thing is, that I think in a lot of ways, Don hates his job, and he has some perspective. He has some integrity. That's what separates him from Cooper and Sterling. His obsession with his job is really a cover, so he doesn't have to think about the more painful aspects of his life.