I remember I got nausea riding in my coworkers floaty Lexus LS. It seemed float like a boat on water. Maybe something was wrong with the suspension? Although there was considerably less leg room in his 3 series BMW and the ride was jarring, I'd rather be uncomfortable than to be sick. In other words, my experience is contrary to most of the replies on here. A sway bar and stiffer springs would help. Thinner tires would not. You should check with your wife, whatever it is specific that is bothering her before any upgrade.