The above code is good, cause it uses CSS, which is a good way to apply the style to your pages (maybe you already know a lot about CSS, in that case sorry to repeat).
It is best to use an external CSS stylesheet that you link to, cause then it's just easier to read your code.
For aligning the page in the center of the screen, I've found that the way described above, using align:center; is great. However, IE6 sometimes doesn't like it. Because of that, I usually do the following to align my pages:
body {
text-align:center;
}
site_wrapper {
margin:0px auto;
}
That code would go in your external CSS stylesheet, and then on your HTML page you put the following:
<div id=site_wrapper>
<!*** site goes here **>
</div>