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