Some people say they need dual 30" monitors to code, I'm not going to disagree with their personal preferences. But other people (like myself) see zero difference in productivity between that and a 13" Macbook Air screen, which I've been coding on for years.
The whole "real machine" thing is just ridiculous cowboy posturing, unless you're doing video game development or something. A Macbook Air is just fine for web development.
If you code all day, you will probably develop back problems if you don't sit with good posture.
You need to sit straight in a good chair, and look at a monitor at eye level about 2 arms lengths away. You need to type on a keyboard with good typing posture.
If you don't, then you will be okay for a few years, and then you won't.
I've seen RSI write off good programmers. It's pretty horrible - they build their lives and self esteem around a job, and then they can't do it. Bad RSI means that you can't touch a keyboard.
Also, I suspect that this is a problem if 80% of your time you are at a desk typing, but if you move around a lot, it's not going to be such a problem.
That said, if you are coding at a desk all the time, a quick google finds research that backs up the posture -> injury thing.