You're dealing with a few factors here -- browser differneces, user monitor resolutions, operating system quirks, installed font sets, and possible end-user css overrides. You can't control all of these things, so you have to control what you can. A couple of tips that many experienced web-commerce coders follow --
if you're going to absolute-position everything, design for 800x600. Unless your target market is hardcore geeks, 70% of web shoppers still use 800x600.
use external css. Not only does it push you to think in a big-picture sort of way, it makes later site updates/changes easier to implement.
test it! Opera, Firefox, and IE are all free. We test, in-house, every site we code in the two most recent versions of each of those three browsers, plus safari and konqueror. We run tests from Windows XP, 2000, Mac os X and 9., and two linux systems. We test at resolutions from 800x600 up to 1600x1200 (except for handhend apps of course.) Know what your customers will encounter before they do.
Use a HTML and CSS validator. I like the W3 validators myself.
Learn. There are many good css sites out there. After you have a foundation down, find sites you like, and view their source code and css files, you can pick up some good tricks.