Hair can be one of the most attractive parts of a woman, and I do notice when she's changed it (though sometimes I will have drawn a complete blank on how). It frames her face, tells something of how much effort she put into getting ready, sets off her eyes, and is usually the first signal a guy expects to get that she's interested.
Remember 'Scent of a Woman'? "The hair... They say the hair is everything, you know. Have you ever buried your nose in a mountain of curls... just wanted to go to sleep forever?"
Hair does have an impact; it's just that some guys will comment on it.
About 2 years ago, I realized that the first thing I've always noticed about a girl is her hair. I hadn't given much thought of what it was before that, I assumed it was a cute face, or body. But it's her hair. I prefer long straight hair. Then I look at the rest.
Some guys notice, some really just don't know. Out of the ones who do notice, most probably won't mention it because they think it sounds gay or think the girl might get a wrong impression after one compliment.
It's hard to say with men in general. I think it varies depending on what other features they notice first.
Most of my guy friends notice when I cut or dye my hair, even if it's not drastic, and ask me about it or tell me it looks nice. I think it's something they've just learned to pay attention to because my hair is one of my more prominent features. I'm constantly dying my hair weird colors. And if you look at the pictures on my profile, you'll se this, haha. So I think it's more like, 'what's she going to do next?'
Some guys are clueless to how a girl styles her hair. Most won't notice little stuff, like if you get it trimmed. When you get a haircut, straighten, curl or something along those lines they notice though. I know a lot of guys that notice long hair, because they love it.