Font usage from a design and usability perspective

[Day Two Webdesign]

I'm studying the field of font usage - especially from a web design and usability perspective.
Because you simply cannot use 'any type of font' within the body text of an HTML page (simply because your website visitors probably do not have your great-looking-but-rare font installed on their computers), you have to stick with the use of fonts such as Arial (pc) / Helvetica (Mac), verdana, times new roman, courier, etc.

Throughout a page, you of course have multiple options to vary your font usage in a consistent way.
E.g.:
-Font types: which fonts to use on a page
- Fonts size: big fonts for <h1> column heads , mid sized fonts for <h2> sub headers, and smaller sized fonts for <h3> body text.
- Font color: bright colors, that maybe align with a color from the website logo, for column heads, neutral colors for body text
- Font weight: bolder for a column head, thinner for body text
- Text decoration: italics and underlines, and for the underlines is a solid line best, or perhaps a dotted underline?
- Letter spacing: the amount of space between letters within a word
- Word spacing: the amount of space between words
- Line spacing: the amount of space (vertically) between two lines
- Text column width: to my opinion, a website is in general read like a newspaper, so the best thing to do is to use text columns. But how wide should a column be? Too small is hard to read, and too wide is hard to read as well! So which is optimal?
- etc.

My questions:
1) which other options to vary with do you use or would you recommend?
2) which best practises do you use for optimal information hierarchy throughout a webpage, and why?
3) what is in your opinion the optimal amount of different fonts used on a single page, and why?