i prefer css over table alignments any day, a bit cleaner.

plus, a small tip, validate your pages with Validator.W3.org. Valid pages have a 90% change of displaying correctly on all browsers.

if you are unshure wheter to use 800x600 or 1024x768 build a phpscript that generates different pages based on the user's resolution (lots of work here, don't bother if you don't have the time)