Don't specify font sizes on the web in pixels. A pixel doesn't correspond to anything useful, how big it is depends on the monitor and resolution, and some browsers (IE6 and IE7 particularly I think) will refuse to resize text with an explicit pixel size.
Specifying in points is theoretically better, because a point is a real size - 1/72nd of an inch - but for this to work the user's machine must be set up correctly to know how many pixels the screen has per inch. Most aren't.
Best to leave the size unspecified, and then it will come out as whatever the user has chosen in the browser. And again, most don't bother, but at least lots of sites will look too big or too small for them, so they'll hopefully realise it's something about their system rather than the forest site.
Where text in different sizes is needed for headings etc, use percentages. Then these will always be the same multiple of the default size used for the bulk of the text.
Neil - line height will set the spacing between lines of text, not the size of the font itself. But you're right to suggest using ems. One em is equal to the height of the font, whatever that may be, so it's a good unit to use when sizing other bits of the layout relative to the text.